I think "to be" has nothing to do with "must"
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).
Further its another use is that using infinitive in such type of sentences is generally seen in Paper's headlines or at the time of giving advise or planned events. See this link
(
Learning English | BBC World Service)
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. Quote:
Originally Posted by Harry Smith  Hi, Svartnik!
The verb "to be to" is the equivalent of the modal verb "must". It's used in timetables, schedules etc.
e.g. The train is to arrive soon.
I'm to be at university at half past eight.  |