Choose the correct:

'I can't believe I got the final question in the pop quiz wrong.'

'I (wouldn't / won't) have known the answer either'

A book says that wouldn't is the only correct option. But in the same book: enter image description here

Is it a mistake in the book? Why won't is not correct?

  • What's the book's title, please?
    – Victor B.
    Jan 17, 2020 at 15:41
  • @VictorB Destination C1/C2 Grammar and vocabulary, Macmillan Jan 17, 2020 at 15:49
  • '[f I had been asked that question] I would not have known the answer.' VS 'They will not have heard the news, will they?' Jan 17, 2020 at 16:19

2 Answers 2


Wouldn't have is hypothetical. Won't have is real.

"I can't believe I got the final question in the pop quiz wrong."

"I wouldn't have known the answer either."

The first person took the quiz, and they can't believe they got the final question wrong. The second person would have gotten the question wrong too if they had taken the quiz (instead of the first person). The second sentence is hypothetical because it was the first person who took the quiz, not the second. The second person is simply imagining what would have happened if they had taken the quiz. There might of course be other interpretations, depending on context.

They won't have heard the news, will they?

As your book says, will have is used to express certainty about the past. There is nothing hypothetical in the sentence above. In this meaning, will have is close to must have, with must also expressing certainty. It doesn't necessarily refer to the future; it acts as a modal verb of probability or expectation.

Practical English Usage, 39.7:

We can use will to talk about the present – to say what we think is very probably or certainly the case.

'There's somebody at the door.' 'That'll be the electrician.'
Don't phone them now – they'll be having dinner.

Will have ... can express similar ideas about the past.

As you will have noticed, there is a new secretary in the front office.
It's no use expecting Barry to turn up. He'll have forgotten.

Meaning and the English Verb:


Usually will with this meaning makes reference to the future, but there is also a kind of ‘prediction’ that refers to the present or past:

By now they’ll be eating dinner [looking at one’s watch].

That’ll be the electrician – I’m expecting him to call about some rewiring [on hearing the doorbell ring].

They’ll have arrived home by now. (Note the use of the Perfect here.)

In By now they’ll be eating dinner, the speaker makes a ‘forecast about the present’, based on previous experience, concerning an event not directly observable. In the same way, someone who says That’ll be the electrician ‘predicts’ the identity of someone at that moment invisible.

To this extent, will (= ‘prediction’) belongs to contexts similar to those of must (= ‘logical necessity’). In fact, must could replace will in all three examples above with little change of effect.

The Cambridge Grammar of the English Language, Chapter 3, 9.5.1:

Will has the same semantic strength as must, and in its central-epistemic use it can generally be replaced by must with relatively little change of meaning. Must conveys the idea of conclusion, and is often used in explanations: Ed’s late – he must have overslept. With central-epistemic will it is more a matter of assumption or expectation, very often with a suggestion of future confirmation, as in: I can’t tell you what the word means but it will be in the dictionary. Compare futurity You will find it in the dictionary. Will would therefore hardly substitute for must in You were mad to tell her: you must have known how upset she would be, for it is hardly a case where the issue of future confirmation would arise.

  • That is correct! Thank you. Wouldn't have is hypothetical - that is what I have missed. Jan 17, 2020 at 17:19

This is a case of the so called Mixed conditional in speech. The first sentence means that Person 1 didn't understand the question that was asked in the quiz. Person 2 replies to Person 1, while using the so called Mixed conditional of the type: if + 2nd conditional / 3rd conditional. A grammatically complete sentence, which is implied, is that: 'If I got the final question in the pop quiz right, I wouldn't have known the answer either.' This Mixed conditional informs Person 1 that it was impossible for Person 2 to answer any quiz question at all. If we put the Future Perfect Tense clause instead of 3rd conditional, then it is not a case of general condition with an imaginery result. It becomes an - if sentence, where Person 2 has to know what the content of the question in pop quiz was so as to say 'I will not have known it either.' Because there is no such information, the construction with the Future Perfect tense for the main clause is impossible. So, the authors of textbook are right: the construction 'won't have known' is wrong in such a dialogue.

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