[BobK]

Originally Posted by rajan

I think "to be" has nothing to do with "must"

But the thread is not discussing "to be"; it's discussing "be to". See Harry's examples.

Originally Posted by rajan

I argue as :
I am supposed to be at the university half past eight
Like wise, The train = to arrive soon. (replace "is with "=").
I think usining infinitive creates less possibility of occuring something than using tense.
1) The train will arrive soon (More Definiteness)
2) The train is supposed to arrive soon / The train is to arrive soon (Less Definitiveness).

I don't follow. Perhaps you need to look at the punctuation again. But anyway, I disagree with the last example. 'The train is to arrive soon' isn't less definitive. It's referring to a schedule.

Originally Posted by rajan

Further, [its] another use is that using an infinitive in such [type of] sentences is generally seen in newspaper headlines or when giving advice or schedules for planned events. See this link
(Learning English | BBC World Service)

Originally Posted by rajan

Yes these kind of sentences can be used in timetables, schedules, official arrangements then it does not mean it is less certain but it gives the meaning of must is not clear to me.
In the example, I am supposed to be at the university ..... ( Here to be is functioning as a linking verb, not as a auxiliary)
If I am wrong, please correct me.

Again, I'm lost. The first string of words isn't a sentence. I think it may need to be two or three: How about this:

'Yes, sentences like these can be used in timetables, schedules, and official arrangements . In contexts like that it does not carry a suggestion of uncertainty. But I don't think it has the same meaning as "must".'

b