Wah How to use "Supposed to".

We can use supposed to to indicate obligation.
We usually use this form in an unfulfilled or angry way.

Be + Supposed to + Verb

It is similar to should and ought to.

• You are supposed to be at work. (You should be at work.) 
• You are not supposed to smoke here. (You should not smoke here.)
• I am not supposed to be here. Please don't tell anyone.
• You are supposed to introduce yourself.
• Are you supposed to know that? 
• What am I supposed to do?

We can use supposed to to indicate obligation in the past.

• I was supposed to go to work yesterday. (But I didn't.)
• How was I supposed to know that? (No one told me about that.) 
• You were supposed to clean your room.
• He was not supposed to tell you.
• Sally was supposed to meet her friend last night.

They are supposed to study hard for the exam tomorrow.