to be infinitive

1. give formal instructions or orders

You are not to leave this house without my permission. Is that clear?
(You must not leave this hourse without my permission. Is that clear?)

2. refer to future events.

A man is to appear in court later this morning charged with the murder of the footballer, Darren Gough.

3. We often use be to + infinitive in the if-clause in conditional sentences when talking about preconditions for something to happen.

If we are to catch that train, we shall have to leave now.

4. be to + perfect infinitive is sometimes used to show that a planned event did not materialise:
He was to have appeared in the West End show but broke his collar bone during rehearsals.


See details: http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv103.shtml