Expectations: Be supposed to and Be to

Examples:

(a) The game is supposed to begin at 10:00.
(b) The game is to begin at 10:00.
(c) The committee is supposed to meet tomorrow.
(d) The committee is to meet tomorrow.

Be supposed to and be to (a form of be followed immediately by an infinitive, e.g., is to begin) express the idea that someone (I, we, they, the teacher, lots of people, my father, etc.) expects something to happen. Be supposed to and be to often express expectations about scheduled events or correct procedures.
In (a) and (b): The speaker expects the game to begin at 10:00 because that is the schedule. Be to is stronger, more definite, than be supposed to.

Compare

(e) I should go to the meeting. I can get some information if I go. Going to the meeting is a good idea.
(f) I am supposed to go to the meeting. My boss told me that he wants me to attend.

Be supposed to and be to also express expectations about behavior; often they give the idea that someone expects a particular person to do something.

Be supposed to is close in meaning to should, but be supposed to, as in (f), gives the idea that someone else expects (requests or requires) this behavior.

Compare

(g) I must be at the meeting. The meeting can't occur without me because I am the only one who has certain information.
(h)  I am to be at the meeting. My boss ordered me to be there. He will accept no excuses.

Be to is close in meaning to must, but be to, as in (h), includes the idea that someone else strongly expects (demands or orders) this behavior.

Be to is used to state strong expectations: e.g., rules, laws, instructions, demands, orders.


Cheers!
Naeem