The use of "will" in assumptions

We can use will for both past and future assumptions. But how do we differentiate if it refers to past assurance or future possibility? For example:

You will know all about Rachel, of course.

Now this conveys two possible meanings 1.) It is a past assumption and the speaker implies that the person knows about Rachel for sure. 2.) It could also mean that the person will know about Rachel at some point in the future.

Another example:

He will have finished eating supper now.

So how do we know if it refers to the past or the future? Do we need to change our intonation while speaking?

Solution 1:

If I am given a piece of paper with the words You will know all about Rachel, then I cannot be sure if the writer is referring to my present or future knowledge of Rachel. So, the sentence by itself is ambiguous. And you could construct numerous similar examples based on the inhererent ambiguity of the various verb tenses (including modals) in decontextualised messages.

But everyday communication is set in a context and such ambiguity is rare. In the case of your example it is difficult to imagine a natural context where any ambiguity could arise. As to the intonation, if you stress will then it can refer only to your future knowledge of Rachel. For example:

• You will know all about Rachel, because I'm going to tell you now.

Solution 2:

The situation in which a sentence such as your first example might occur dictates the meaning. It can only be (1). For it to mean (2) it would have to be, for example, You will know all about Rachel in due course.

Your second example shows will being used to express prediction. The sentence would normally conclude not with your deleted now, but with by now, or some other time reference, such as by 8 o’clock.

Solution 3:

The only way to really know is to be part of the context in which the sentence was spoken. Because the words don't exist in a vacuum.

If you were Rachel's ex or someone really close to her, and somebody told you: "You will/ would know all about Rachel, of course," then it means: "I cannot doubt whatever intimate knowledge of Rachel you have because you are the best, truest, most reliable source."

The same goes for your second example:

A. finished action at some point IN THE FUTURE

(When we finally arrive), they will have prepared everything.

B. already FINISHED action

(By now), they will have prepared everything.