'must' for probability

[Explorer]

Thank 5jj and Tdol for the answers.

Sorry, but I don't understand your question. He must have done that yesterday is fineIt can.

My apologies, I used incorrectly 'the circumstances of time' instead of 'adverbial modifier of time'.

 

[Esredux]

Perhaps "logical deduction" rather than "logically deduced certainty"? I'm not sure it always expresses certainty. I actually think it can be found in sentences expressing uncertainty (without any irony).

I don't get it. With all the balloons and the funny hats and stuff, it must have been a lot of fun, right? How come then my son won't speak to me? He's acting like I've done something wrong.

Here, the speaker is deducing something, but they are not certain their deduction is correct. They see there's a problem they don't understand, and they accept that their may be a flaw in their deduction.

I am having a hard time trying to accept 'must' for 'uncertainty'. Would it make sense to say that in 'it must have been a lot of fun' the speaker expresses 'logically deduced certainty' whereas in 'right?' (with a rising intonation) he is seeking confirmation like it suddenly struck him that he might have missed something? I should think the pause between the two parts is more for a full stop rather than for a comma.

I have no problem with the example as such. It looks very much like a series of fragmentary utterances alternated with some unexpressed thoughts that can be easily (imo) reconstructed, something like:
'I don't get it... (I have invested so much time/money/effort in this party..) with all the balloons and the hats.... and all that stuff..(because I was sure he would like it) It must have been a lot of fun (that's what all boys of his age usually like) (what do you think?) Right? (if my reasoning was right then I really don't understand) how come my son won't speak to me (even worse than that) he's acting like I've done something wrong'

If this piece sounds convincing, then I'd say what makes the poor father suffer is actually the clash between what he was sure was the right thing to do and the reaction of his son that he is struggling to understand. If so, then here 'must' expresses 'certainty based on some, quite possibly wrong, yet logical deduction'.

ps. Funnily enough, I was sure it should have been the boy's father speaking (wrong assumptions ;-)) Clearly, it could have been his mother as well.