'must' for probability

[Explorer]

Is it correct to use 'must' for expressing probability in sentences like

'He must return, he's forgotten his umbrella.' (He will probably return, he's forgotten his umbrella)
'He's trained much, he must win the game'. (He's trained much, he will probably win the game)

[abaka]

"Must" is indeed sometimes used to mark probability in certain informal contexts. Examples:

Given how well the Netherlands has been playing, they must surely win the championship this year.

He has many umbrellas. He must have a hard time picking one.

But it's not a good usage if there is the least possibility of confusion with the usual "must" of obligation or necessity. Therefore your second sentence is perhaps acceptable, although it should be avoided in formal writing. Your first sentence is very questionable, I think.

[charliedeut]

Given how well the Netherlands has been playing, they must surely win the championship this year.[/I]

Which championship are you talking about? Surely not the Euro2012, they could have hardly done worse there

[abaka]

[5jj]

The use of 'must' to indicate logical certainty is not particularly informal.

It is not generally used if there is a possibility of ambiguity. When it is clear that the present moment is being spoken of, then there is no problem; 'must's uggesting obligation almost always applies to something that must be done after the moment of speaking.

'He must return, he's forgotten his umbrella.' (not for logical certainty)
'He must be very wet; he's forgotten his umrella.' (fine for logical certainty)
'They must win tomorrow, or I'll lose my bet.' (not for logical certainty)
'They must win tomorrow after all the money they've spent on new players'. (possibly ambiguous)

[Explorer]

One question more in this topic.
The sense of which expression of the two ones placed below is closer to the sense of 'He must be very wet...'
'He is certainly very wet...'
'He is probably very wet...'
?

[5jj]

It's probably closer to 'certainly'. Will suggests more absolute certainty than must, which suggests a logically deduced certainty; that is stronger than probability.

[Explorer]

Thank you very much for your answer.

Please check the following examples whether they contain any ambiguity. In all of the sentences the speaker expresses certainty with respect to future events.

'His car is moving 10 Km/h faster, he must come up with Jon in an hour.'
'She dresses brightly, he must notice her right after he comes the hall.'
'She understands that her arguments are weak, she must change your opinion.'
'He has a strong influence on the community, they must accept his proposal after the first reading.'
'- I am afraid this problem may upset the plans. - Don't wory, Eddy is an inventive guy, he must arrive at a solution quickly.'

[birdeen's call]

It's probably closer to 'certainly'. Will suggests more absolute certainty than must, which suggests a logically deduced certainty; that is stronger than probability.

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.

[5jj]

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 see your point, but am not totally convinced. I think the speaker is very nearly certain (not absolutely certain - that's will). Given that near-certainty, they cannot understand why their son is unhappy. There is not necessarily a flaw in their deduction. The party may have been fun for everybody except their son. If that's the case, they want to know why.

One point about logically deduced certainty (or even absolute certainty) is that it is certainty, not fact, expressed by the speaker:

A: You co-authored a book with Arthur Nicholls once, didn't you?

B1: Yes, that was in 1969. (I remember that clearly. It was the year my daughter was born. My wife did some proof-reading in the maternity ward! Apart from that, I have shamelessly cited the book whenever I can, so 1969 is fixed in my mind.)
B2: Yes. That will have been in 1969. ( It was the last year we shared a flat together, and I am certain that that was 1969)
B3: Yes That must have been in 1969. (That was while we still sharing a flat. I left the following year to get married. I got married in 1970, so I am now certain, after logical deduction, that we wrote the book in 1969).

I readily concede that there is not a clear-cut difference between will have and must have in my examples, and that must have is possible for B2 and will have is not impossible for B3. My point is that must have is, in my opinion, closer to the idea of certainty than it is to the idea of probability and that, for each individual speaker, will have is more absolutely certain than must have.