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The moldy melon that changed medicine: the third conditional for an imagined past

This B2 ESL lesson plan teaches the third conditional through the moldy cantaloupe from a Peoria, Illinois, market that helped mass-produce penicillin.

Grammar
If + had + past participle, then would have + past participle (If nobody had brought in that melon, the lab wouldn't have found the mold.), plus might have, questions and 'd.
Topic
How a USDA lab in Peoria hunted for a better penicillin mold, and why the melon's finder is still debated, with words such as strain, vat and by-product.
Activities
An explainer to read, an office argument and a reader's voice memo to hear, self-checking drills, a history club chat and “what if” boxes.

What's inside

  1. Chapter 1Warm-up: lucky finds

    • PictureUnlock
    • Spin the wheel and answerSpin the wheelUnlock
    • Study the words: from the market to the labVocabularyTry it
    • Look at the pictures and choose the wordChoose by pictureUnlock

  2. Chapter 2Let's read: one melon, one mold

    • Look at the photo: a melon at the marketPictureUnlock
    • Read Leona's explainer on The Why DeskArticleUnlock
    • Read and mark true or falseTrue or falseUnlock
    • Read again and choose the correct answerQuizUnlock
    • Match the dates and the eventsMatchUnlock

  3. Chapter 3Let's listen: luck or a plan?

    • Listen to Leona and IsaiahListeningTry it
    • Transcript, for the teacherUnlock
    • Listen and choose the correct answerQuizUnlock
    • Listen to the Reader's Rebuttal: Ella's voice memoListeningUnlock
    • Transcript, for the teacherUnlock
    • Listen and mark true or falseTrue or falseUnlock

  4. Chapter 4Grammar: If nobody had brought in that melon

    • Study the scheme: the third conditionalGrammar schemeTry it
    • Choose the correct optionChoose the optionUnlock
    • Fill in the gaps: use had or would have and the words in bracketsFill in the gapsTry it
    • Fill in the table: imagine the opposite pastFill in the tableUnlock
    • Choose the correct answer: which sentence is correct?QuizUnlock
    • Put the words in the correct orderUnjumbleUnlock

  5. Chapter 5Let's talk: what if…?

    • Choose the right replies: a history club's “what if” nightChatTry it
    • Open a box and answerOpen the boxUnlock

  6. Chapter 6Homework

    • Match the words and the meaningsMatchUnlock
    • Unscramble the wordsAnagramUnlock
    • Fill in the gaps with the lesson's wordsFill in the gapsUnlock
    • Find the matching pairs: words and meaningsMemory gameUnlock
    • Choose the correct option: new sentencesChoose the optionUnlock
    • Fill in the gaps: new sentences with had or would haveFill in the gapsUnlock
    • Choose the correct answer: does 'd mean had or would?QuizUnlock
    • Put the words in the correct order: new sentencesUnjumbleUnlock
    • Choose the correct option: Leona answers EllaChoose the optionUnlock
    • Drag the words into the gaps: Isaiah's “What if?” postDrag and dropUnlock

33 exercises in 6 chapters. Start a 7-day free trial to open all of them, copy any part into your own lessons, and teach the lesson live.

Science vocabulary

12 words and phrases from the lesson, each with a picture and audio in edio.

+9 more with pictures and audio in the lessonMore words

  • mold
  • cantaloupe
  • bacteria
  • antibiotic
  • strain
  • technician
  • vat
  • by-product
  • microbe

Grammar: the third conditional: if + had done, would have done

The third conditional imagines a past that never happened and the result it would have had: If nobody had brought in that melon, the lab wouldn't have found the mold. Both halves are about the past, and neither is true. With a discovery that hung on one market melon, learners keep both halves in the past, then add softer results, questions and the short form 'd.

If nobody had brought in that melon…

Use the third conditional to imagine a different past and its result. Both parts are about the past, and neither of them really happened.

If + had, then would have + past participle

The if part takes had or hadn't + past participle; the result takes would or wouldn't have + past participle: If Fleming's mold had grown well in the vats, the lab wouldn't have needed a new one. The form is the same for every person: they had used.

The pattern

If + had / hadn't + past participlewould / wouldn't have + past participle

If nobody had brought in that melon, the lab wouldn't have found the mold.

If Fleming's mold had grown well in the vats, the lab wouldn't have needed a new one.

Every person: the same form

PersonIf… (had + past participle)…the result (would have + past participle)
IIf I had seen the melon,I would have taken a photo.
youIf you had eaten it last week,you wouldn't have found the mold.
heIf he hadn't asked the Army for help,he wouldn't have received the samples.
sheIf she had kept the melon at home,she would have thrown it away.
itIf it hadn't sat in the fridge for two weeks,it wouldn't have spoiled.
weIf we had cleaned the fridge,we wouldn't have found this story.
theyIf they had used shallow dishes,they wouldn't have mass-produced penicillin.

Might have, could have and the if part second

When a result was only possible, use might have or could have: If the Army hadn't helped, the lab could have waited years. The if part can also come second, with no comma before if: The lab wouldn't have found the mold if nobody had brought in the melon.

Softer results: might have, could have

Not sure about the result? Use might have (maybe) or could have (it was possible) instead of would have: If nobody had brought in that melon, the lab might have found another strain. If the Army hadn't helped, the lab could have waited years for a good mold.

The if part can come second

Put the result first and the if part second, with no comma: The lab wouldn't have found the mold if nobody had brought in the melon. The meaning doesn't change.

Questions, short answers and would've

Questions put would before the subject: What would have happened if nobody had brought in the melon? Would the lab have found another strain? – No, it wouldn't have. In speech, would have sounds like would've, but never write would of.

Questions and short forms

FormExample
Wh- questionWhat would have happened if nobody had brought in the melon?
Yes / no questionWould the lab have found another strain?
Short answersYes, it would have. / No, it wouldn't have.
Short formsIf she'd found it (she had), they'd have tested it (they would).
would'veWe would've called you. (would've = would have; never write "would of")

In the office and in Peoria

  • If you'd eaten it last Monday, it wouldn't have spoiled.
  • What would have happened if nobody had brought in that melon?
  • If that melon had stayed at the market, a mold from bread might have done the job.
  • If the lab had kept using shallow dishes, it wouldn't have made much penicillin.
  • If the lab hadn't had a plan, would anyone have noticed that melon?
  • Pascal, if you'd cleaned the fridge, we'd never have found this story.

'd: had or would?

In the if part, 'd means had: If you'd eaten it last Monday, it wouldn't have spoiled. In the result, it means would: we'd never have found this story. A participle after 'd means had; have after it means would.

'd: had or would?

In the if part, 'd means had: If she'd found it = If she had found it. In the result, 'd means would: they'd have tested it = they would have tested it. Quick check: 'd + past participle → had; 'd + have → would.

Common mistakes

Three slips break the pattern: would in the if part (If she would have found the melon), a missing have (they would found the mold) and the past simple for an imagined past (If Isaiah ate the melon last week). In standard English, would stays out of the if part.

  • If she would have found the melon, she would have told the lab.

    If she had found the melon, she would have told the lab.

    in standard English, no would in the if part

  • If she had found it, they would found the mold.

    If she had found it, they would have found the mold.

    would + have + past participle

  • If Isaiah ate the melon last week, it wouldn't have spoiled.

    If Isaiah had eaten the melon last week, it wouldn't have spoiled.

    an imagined past: had + past participle, not the past simple

  • The lab would of found another strain.

    The lab would have found another strain.

    would've sounds like "would of", but we write have

More lessons on this grammar: Third and mixed conditionals lessons and exercises

Try the third conditional exercises

Try them yourself: these are the same exercises your students do in edio.

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What's in the lesson

  • 18 auto-checked exercises
  • 6 games
  • 1 article
  • 1 grammar scheme
  • 1 set of word cards
  • 2 audio clips
  • 2 pictures
  • 2 transcripts

The main objectives of this lesson are to:

  • imagine a different past and its result: If Fleming's mold had grown well in the vats, the lab wouldn't have needed a new one.
  • use had in the if part and would have in the result, for every person: If we had cleaned the fridge, we wouldn't have found this story.
  • soften a result with might have or could have: If that melon had stayed at the market, a mold from bread might have done the job.
  • ask and answer “what if” questions: What would have happened if nobody had brought in that melon? Would the lab have found another strain? – No, it wouldn't have.
  • tell 'd for had from 'd for would: If she'd found it, they'd have tested it.

Lesson 5 opens Health breakthroughs, unit 2 of English B2.1 · How the World Works. It joins the second conditional of B1.2 lessons 1 to 3 with the past perfect that lesson 1 used in stories, and lesson 6 goes on to the past perfect and past continuous passive. A breakthrough that turned on one moldy melon raises the question every third conditional asks: what if it had gone differently?

Students spin a wheel of questions about lucky finds, credit and old food in the fridge, then learn twelve words, from spoil and isolate to vat and microbe, with pictures and audio. They read Leona's explainer, which sorts the facts from the legend, then hear Leona and Isaiah argue over a cantaloupe forgotten in the office fridge and Ella, a reader from Illinois, give the credit to the scientists; both are AI-voiced. The grammar chapter has the scheme and five self-checking drills, among them a table that imagines the opposite past. In the speaking chapter, students answer a history club's “what if” questions in a chat and open boxes about luck, plans and credit.

All ten homework tasks check themselves: a match, an anagram, gaps with the lesson's words and a memory game; then form choices and gaps in new sentences, a quiz on what 'd means and word order; then Leona's answer to Ella with forms to choose and Isaiah's “What if?” post with phrases to drag in.

Grammar:
third conditional · conditionals · imagined past · would have + past participle
Topic:
penicillin · Peoria, Illinois · the moldy cantaloupe · the USDA lab · health breakthroughs · the Illinois State Microbe · The Why Desk

Teaching tips

  • Draw two columns, “what happened” and “what if”, before the scheme. Under the first, write Raper asked the Army for help. Students fill the second: If Raper hadn't asked, the lab wouldn't have received the samples.
  • Ask about lucky accidents before the reading: has anyone found something useful by mistake? Students then read to sort the penicillin story into the parts that were planned and the parts that were luck.
  • Play the office argument twice: first to find out who thinks the melon was luck, then to catch every would have and might have. Students explain why Leona answers Isaiah with one word: Might.
  • Mark every 'd in the dialogue: If you'd eaten it, if you'd cleaned the fridge, we'd never have found. Students write each one in full and say which rule decided it.

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The whole course: English B2.1 · How the World Works

32 lessons for adults at B2 in American English, each answering one question about how the world works. The team of The Why Desk, a small online magazine in Pittsburgh, takes on one question a week, from the day a computer beat the world chess champion to eight people sealed inside Biosphere 2. Learners tell stories with narrative tenses, speculate about the past and the future, and report what experts claim. The practice exercises check themselves.See the course

  1. Unit 1 AI and the future of work

    narrative tenses, by and until, be bound to and be likely to, the future perfect

    1. Lesson 1Checkmate: the day a computer beat the championNarrative tenses: past simple, past continuous and past perfect
    2. Lesson 2The robot car that crossed the desertTime prepositions: by, until, within, throughout, in time and on time
    3. Lesson 3Can a chatbot fool you?Probability about the future: be bound to, be likely to, the chances are
    4. Lesson 4By 2035, what will AI have designed?The future perfect simple: will have + past participle

  2. Unit 2 Health breakthroughs

    the third conditional, the past perfect and past continuous passive, have something done, the passive with two objects

    1. Lesson 5 · you're hereThe moldy melon that changed medicineThe third conditional: if + had done, would have done
    2. Lesson 61.8 million Polio PioneersThe past perfect passive and the past continuous passive
    3. Lesson 7One clinic, many doctorsHave / get something done
    4. Lesson 8A kidney from a twinThe passive with two objects

  3. Unit 3 Moving people

    the past perfect continuous, the passive infinitive and -ing form, it's worth and there's no point, the future in the past

    1. Lesson 9Six years to the golden spikeThe past perfect continuous vs the past perfect simple
    2. Lesson 10Riding a pod with no driverThe passive infinitive and the passive -ing form
    3. Lesson 11The town that painted bike lanes firstIt's worth, there's no point and it's no use + -ing
    4. Lesson 12The highways that were supposed to take 13 yearsThe future in the past

  4. Unit 4 Wildlife comebacks

    reporting verbs with a that clause, with an object and infinitive and with -ing, verbs of perception

    1. Lesson 13Did wolves really change Yellowstone's rivers?Reporting verbs + that clause
    2. Lesson 14Bringing a parrot back from the edgeReporting verbs + object + (not) to + base form
    3. Lesson 15The beavers that came by parachuteReporting verbs + -ing and verb + preposition + -ing
    4. Lesson 16Fifteen bison take the trainVerbs of perception + object + base form or -ing

  5. Unit 5 Pop culture goes global

    comment adverbs, wish and if only for the present and the past, have someone do and get someone to do

    1. Lesson 17The party that started hip hopComment adverbs: apparently, supposedly, undoubtedly
    2. Lesson 18Slide to the left!Wish and if only for wishes about now
    3. Lesson 19If only they'd kept SupermanWish and if only + past perfect for regrets
    4. Lesson 20The hit factory on West Grand BoulevardCausative have and get: have someone do, get someone to do

  6. Unit 6 Money and smart choices

    -ever words and no matter, should have and could have, prepositions for numbers and change, it's time and would rather

    1. Lesson 21The bank that never closed-ever words and no matter
    2. Lesson 22One marshmallow now or two later?Should have, shouldn't have and could have
    3. Lesson 23Benjamin Franklin's 200-year giftPrepositions for numbers and change
    4. Lesson 24Why buy a drill you'll use once?It's (high) time and would rather + past simple

  7. Unit 7 Language and communication

    must have, might have and can't have, verb prefixes and suffixes, past modals in continuous and passive forms, agreement with quantities

    1. Lesson 25How did one man write down a language?Past modals of deduction: must have, might have, can't have
    2. Lesson 26Why do Americans write “color”?Verb prefixes and suffixes: misspell, rewrite, simplify, shorten
    3. Lesson 27The joke that needed a smileyPast modals in continuous and passive forms
    4. Lesson 28Can preschoolers save a language?Subject-verb agreement with quantities and collective nouns

  8. Unit 8 Exploring the extremes

    dimension nouns, the future continuous and future perfect continuous, compound adjectives, only, even and just

    1. Lesson 29A walk on the ocean floorDimension nouns and phrases: depth, width, height
    2. Lesson 30Six months of night at the South PoleThe future continuous and the future perfect continuous
    3. Lesson 31Flying on pedal powerCompound adjectives with numbers and participles
    4. Lesson 32Two years under glassFocus adverbs: only, even, just and at all

Questions about this lesson

What is the third conditional?
The third conditional imagines a different past and its result, so neither half really happened: If nobody had brought in that melon, the lab wouldn't have found the mold. In fact, someone did bring it in, and the lab did find the mold. Speakers use it for regrets, lucky escapes and “what if” history. The B2 lesson The moldy melon that changed medicine on edio.ai teaches it with the cantaloupe that helped mass-produce penicillin.
How do you form the third conditional?
Put had + past participle after if and would have + past participle in the result: If Fleming's mold had grown well in the vats, the lab wouldn't have needed a new one. The form is the same for every person. Questions start with would: Would the lab have found another strain? For a result that was only possible, use might have or could have. The scheme in The moldy melon that changed medicine on edio.ai shows all seven persons.
What are some examples of the third conditional?
Four from The moldy melon that changed medicine on edio.ai: If Raper hadn't asked the Army for help, the lab wouldn't have received several thousand mold samples. If you'd eaten it last Monday, it wouldn't have spoiled. If that melon had stayed at the market, a mold from bread might have done the job. What would have happened if nobody had brought in that melon? Each one changes a past fact and imagines what would have followed.
What's the difference between the second and third conditional?
The second conditional imagines the present or future; the third imagines the past. Compare If I found a moldy melon, I would throw it away with If I'd found that melon, I would have thrown it away. The second takes the past simple and would + base form; the third takes had + participle and would have + participle. On edio.ai, the second comes in B1.2 and the third in The moldy melon that changed medicine.
Is it “if I would have known” or “if I had known”?
It's if I had known: in standard English, the if part takes had, not would have. So If she would have found the melon becomes If she had found the melon, she would have told the lab. Would have belongs only in the result. That slip heads the list of common mistakes in The moldy melon that changed medicine, a B2 lesson on edio.ai, and the wrong replies in its chat test it again.
Did Mary Hunt find the penicillin cantaloupe?
Nobody knows for sure. Mary Hunt, a technician at the USDA lab in Peoria, was thanked in a 1944 paper by the lab's scientists for collecting moldy samples, and local newspapers later credited “Moldy Mary” with the melon. But in 1976, Kenneth Raper, who ran the lab's mold collection, said a Peoria woman handed it to a guard and left without giving her name. The USDA has no clear answer, as The moldy melon that changed medicine on edio.ai explains.
How did a moldy cantaloupe help mass-produce penicillin?
Around 1943, a cantaloupe from a Peoria market reached the USDA lab there, which needed a mold that thrived in deep vats. Its mold made far more penicillin than Fleming's: about 60 micrograms per milliliter against about 1.2, a 2012 science review says. Improved with X-rays and ultraviolet light, it became the parent of the strains used to make penicillin. In 2021, the melon mold became Illinois's State Microbe. The moldy melon that changed medicine on edio.ai tells the story.
How do I teach the third conditional to B2 students?
Start from real events with a clear cause and result, then flip each one: Raper asked for help, so the lab got the samples becomes If Raper hadn't asked for help, the lab wouldn't have gotten the samples. Add might have once the full form feels easy. The moldy melon that changed medicine on edio.ai moves from Leona's explainer and the office argument to the scheme, a table that imagines the opposite past and four more drills.
What's a good game for practicing the third conditional?
Try a chain story: one student names a past event, and each next student adds a link: If I hadn't missed the bus, I wouldn't have walked. If I hadn't walked, I wouldn't have found the bakery. For speaking, the boxes in The moldy melon that changed medicine on edio.ai ask questions like Think of a lucky moment in your own life. What would have happened if it hadn't happened?
What's good homework for the third conditional?
Good homework mixes the full form with the short forms: choices like would have gone or would went, gaps with the verb in brackets, and a quiz on whether 'd means had or would. All ten homework tasks in The moldy melon that changed medicine on edio.ai check themselves, and the last one is Isaiah's alternate history of the melon, with phrases to drag in.
How do you practice the third conditional one-to-one?
Trade lucky breaks and near misses: the student tells about a moment that changed their week, and the teacher asks What would have happened if you hadn't…? Then swap roles. The chat in The moldy melon that changed medicine on edio.ai works without a partner: Lucinda tests her history club's “what if” questions, and each wrong reply hides a third conditional mistake.
Are there interactive exercises for the third conditional?
Yes. Without an account, this page plays three parts of The moldy melon that changed medicine, a B2 lesson on edio.ai: Leona and Isaiah's argument over the melon; a gap-fill with had or would have; and Lucinda's “what if” chat. Everything else needs an account.
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