1. It can be used to mean 'to exist':
I think, therefore I am. I exist because I think (Descartes).
2. The progressive form, if it is followed by a present participle (-ing form) :
He is sleeping. He is asleep at the moment.
3. The passive voice, if it is followed by a past participle:
The car was repaired by my brother-in-law. My brother-in-law repaired the car.
4. Obligation, duty, agreed actions (in the future), if it is followed by an infinitive:
You are to do what I tell you to do. You must and will do what I tell you to do.