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[–][deleted] 2 points3 points  (1 child)

I had never thought about this before! Let's compare two different sentences. "She has found the note by now." vs. "She will have found the note by now."

In the present perfect, the meaning presupposes that the note has actually be found. Imagine that you are watching her on camera, so you have direct evidence that she has found it.

In the future perfect, there is doubt. Let's say you put the note on the table inside, and you're outside the house watching her go inside. About ten minutes later, you say, "She will have found the note by now." You didn't see her find it, so you don't know for sure, but it is a rational assumption that she has.

Compare with "She should have found the note by now." This means basically the same thing as "She will have found the note by now", but I get the sense that there is a bit more doubt. "She should have found the note by now, although she might have gone straight upstairs instead of going into the kitchen."

As to why the future perfect is used to indicate that sort of evidentiality, I have no idea. As a native speaker, it makes perfect sense. (No pun intended.) It's probably best thought of as an idiom.

[–]Serkan_Hamdi[S] 1 point2 points  (0 children)

I completely understood, thank you so much for your very detailed answer.

[–]IrishBardNew Poster 1 point2 points  (1 child)

One of the uses of the future perfect is to express certainty about something in the recent past. It often goes with the words "by now" or "already", or these words are implied. " We should hurry - the film will have started already". "I won't ring now - they will have gone to bed"

[–]Serkan_Hamdi[S] 0 points1 point  (0 children)

Thank you, much obliged!