This is from the movie Harry Potter. Harry and a man, who Harry sees for the first time, are talking, and the man thinks that Harry is aware of his own magical powers - that he is a wizard.

Harry: Excuse me. But who are you?

The man: Rubeus, Keeper of keys and grounds at Hogwarts. Of course you will know all about Hogwarts.

Harry: Sorry, No.

The man: No?

In the second sentence, The man says "Of course, you will know .....". This sentence, at first look, might seem a simple future tense because it has "will" as the auxiliary verb.

But when I think about the meaning of the sentence, it does not seem to be about the future. On the contrary, the man is making a strong guess or a deduction about present. He assumes that Harry already knows that he is a wizard, and he must already have had much information about Hogwarts. So, it seems that "will" here is used to make a deduction.

But, this usage is not so common, so I want to make sure whether the "will" in this sentece is simply a future tense, or is it used for deduction about the present (same as "must")?

  • Unfortunately, we all 'know all about Hogwarts'. Jan 29 at 0:27

3 Answers 3


Fascinating. The meaning of that sentence is so obvious to me that I don't even think about the grammar. It's not until someone else asks a question like this that I realise how tricky the structure is.

The opening post is right to identify that will in this context is not a simple future tense. It does NOT mean "You don't know now but you will come to know in the future."

Instead it's talking about the present situation, and as the opening post suggests it has the idea of a strong assumption that a certain fact is true. In the above sentence "you will know" means the same as "no doubt you know". Rubeus thinks Hogwarts is so well known that he can't imagine anyone not knowing about it.

So why is will used here? I think because there is still a future component to the sentence. Suppose a teacher sets homework for his class, then the next day he checks what they have done. He might say, "Let's look at your homework - of course you will all have done it." Here the teacher makes an assumption that the class has all done their homework. Who would dare to disobey his instructions? But he won't know for sure until he's opened each student's book and checked. Will is the future tense that bridges the gap between present assumption and future confirmation.

Although this usage is not as common as a simple future tense, it is an entirely natural expression.

  • Thanks for the good example. About example of teacher saying "...of course you will all have done it" reminds me of using the "must", because it is also used in the sense of strong assumption. So, can the teacher say "...of course you must have done it"?
    – yunus
    Jan 28 at 22:04
  • Not quite. Must has a stronger sense of obligation. "You must do it" (This implies, if you don't do it there will be consequences.) "You will do it" (This implies that I certainly expect you to do it, but no further consequences are implied.) Jan 28 at 22:11
  • No, I mean the use of "must" for assumptions. For instance, you go to your friends house late at night, but from the outside you see the lights are off. So you can say "Oh, the lights are off. He must have gone to bed". This usage of "must" is also strong assumption rather than an obligation, isn't it?
    – yunus
    Jan 28 at 22:17
  • In that sense, yes. But it still feels like there is a difference of emphasis. Maybe the point is that "must" points to the only explanation. ("He must have gone to bed" means there is no other possible reason for the lights being off. "He will have gone to bed" is the assumption I'm making, but it doesn't rule out other explanations.) Jan 29 at 15:12

'Will' can be used as a modal verb to refer to what is likely in the present. The fragment could be rewritten as:

Of course it is very likely that you know all about Hogwarts.

will modal verb (LIKELY) (also 'll)

used to refer to what is likely:

That'll be Scott at the door.
That'll be his mother with him.
As you all will know, election day is next week.
You will have heard about the plans for next week.
You will probably have already made plans for the weekend.
That'll be Tony on the phone.
As you will have guessed by now, David and I are engaged.
You will be aware that things haven't been going well.

Will (Cambridge Dictionary)

  • Regarding the example, "You will have heard about the plans for next week", can we use "must" instead of "will" without changing the meaning? For instance "You must have heard about the plans for the next week.
    – yunus
    Jan 28 at 22:08
  • You can, but it does change the meaning. If you answer "No" to "You will have heard about the plans", the person may be a little surprised, but if you answer "No" to "You must have heard about the plans", they'll be very surprised and might have difficulty believing that you haven't.
    – Colin Fine
    Jan 28 at 23:20

Like all modals, will can be used in an epistemic (about somebody's knowledge or understanding of the world) as well as its more common deontic meaning.

Here it has that epistemic meaning: as you say, it means something like "I deduce that" or "I assume that".

  • So in that case, the "must" in the sense of deduction overlaps with "will". You go to friend's house late at night but lights are off. You can say "Oh, he must have gone out."
    – yunus
    Jan 28 at 22:20
  • Yes. Epistemic must is a stronger deduction: the speaker is more sure of it. Will is more an assumption (I've edited my answer to say "I assume" rather than "I'm sure").
    – Colin Fine
    Jan 28 at 23:17
  • Assume on very good authority, unless there is a questioning inflection? Jan 29 at 0:24

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .