• Obligation arising out of an arrangement or plan/in statements and questions/.

We are to complete this work by tomorrow.

When is the wedding to be?

• A strict order or instruction, given by the speaker or any other authority.

You are to do it exactly you were told.

you are to stay here until I return!

• Possibility.

They are not to be trusted!

Nothing was to be done under the circumstances.

• Perfect Infinitive is used to emphasize that the action did not take place.

I was to have come.

I was to have graduated in June but failed.

• Strict prohibition.

You are not to do that!

You are not to tell anything about it!

• In the expressions:

What am I to do?

What is to become of me?

Where am I to go?

• A purpose or a plan.

A knife is to cut with.

This prize was to honor him for his success.

• Something that is destined to happen.

She was to become his wife.

It was not to be.