How can I use the exact word "will" in a sentence about past? [closed]

Last week our teacher gave us an assignment to find a usage for the word "will" in a sentence about past.

He also told us that it doesn't have anything to do with would and it's rarely used.

Your teacher gives you some interesting puzzles to solve!

Anyway, one of the functions of will is to express confidence or certainty about a situation. This situation can be in the past. So, if you come home after school and your mother says:

Someone phoned this morning, but I couldn't answer because I was in the shower.

You can reply:

Oh, that will have been my teacher. He said he tried to call you, but no luck.

You can use the will+perfect construction in a similar way in sentences such as:

You will have recently received a letter from the school about your son's behaviour in class.

You can impress your teacher by telling him or her that this use of will is called epistemic modality by The Cambridge Grammar of the English Language (p188). It gives the example:

They will have made their decision last week.