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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 use supposed to to say what we expect someone to do or what someone should do.

• 1. You are supposed to clean your room before Mom comes home. (She expects you to clean it.)
• 2. The gift was supposed to be a surprise for Sally, but you told her about it. (It should have been a surprise.)
• 3. The children were not supposed to eat candy before bed. (They should have waited.)

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?

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