|We use be supposed to + verb infinitive to express that something is (or was) expected to be different:
John is supposed to be here with us now – where is he?
(= 'It was agreed that he would be here.')
The party wasn't supposed to finish so early, but most people were tired so they left.
|We can use be supposed to to say that we should do (or should have done in the past) a thing according to a rule or law:
You're supposed to keep your seat belt on during the flight, but I took mine off.
You’re not supposed to drive over 50 m.p.h. in a town – slow down!
You weren’t supposed to say anything about the party – it’s a secret.
|We also use be supposed to to say that a promise or arrangement was not fulfilled.
When used in this context, be supposed to is very similar to should have:
You were supposed to call me this morning. Why didn’t you?
The train was supposed to arrive twenty minutes ago.
(= ‘The train should have arrived twenty minutes ago.’)
|We can use be supposed to in a question to express that we see something as a problem or as impossible:
How am I supposed to get to work on time if the train is always late?
|We can use be supposed to to criticise someone’s performance:
Listen to him – he can’t sing in tune. He’s supposed to be a professional singer, isn’t he?