Wah How to use "You are to"

We can use Subject + Be + To when we are talking about responsibilities or duties.
This form has the same meaning as Be supposed to.

• My mom told me that I was to clean up the house before she was back. (I was supposed to clean up the house.)
• My boss says that John is to finish the job by Friday. (John is supposed to finish the job by Friday.)
• The doctor said that I was to start exercising. (I was supposed to start exercising.)
• Employees are to wash their hands after using the toilet.
• Jane is to present her report first.
• My children are to read a book before bed every day.
• The security guards are to ensure that everything is alright.

The police officer told her that she was to drive slowly.

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.