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What's the difference between...

Gandalf will have killed hundreds of dragons in his time.

Gandalf must have killed hundreds of dragons in his time.

The first sentence is a line from a movie script, and the second one is a something I made up. I've seen a few grammar webpages that says "Will + Have + PP" can be used for past certainty when they are pretty sure about it. (Not future perfect). But how much is it different from "Must + Have + PP"?

  • [something I made up] – Lambie Dec 6 '19 at 21:57
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Gandalf will have killed hundreds of dragons in his time.

Gandalf's time hasn't happened yet. When it does, there's going to be hundreds of dead dragons.

Gandalf must have killed hundreds of dragons in his time.

Something has forced "killed hundreds of dragons in his time" to be true. This is due to something we may or may not know.

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Yes, will have expresses certainty. 'He will have killed...' means 'He has certainly killed...'.

The modal auxiliary must expresses conjecture. 'He must have killed...' means 'He has probably killed...' or 'He has almost certainly killed...'

Since we can't be absolutely certain what others are feeling, must is often used when we are wondering how they feel. For example 'He must have been embarrassed.' 'What must they be thinking?'

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