The verb to be can be used as a linking verb or auxiliary (helping) verb. The helping verb to be combines with main verbs (come, wait, drive, send, break, etc.) to help them express grammatical meanings.
Examples of helping verb to be
I am coming back from my holiday tomorrow morning.
She will be waiting for you at six o'clock.
He was about to drive to work when the storm started.
This email was sent last week.
The window had been broken before we arrived there.
Compare: Linking verb to be
Read more: Verb to be forms and use
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Verb to be as a helping verb
1. The verb to be is used to form continuous tenses and the passive voice.
Present continuous: I am sleeping.
Past continuous: I was sleeping.
Future continuous: I will be sleeping.
Present perfect continuous: I have been sleeping.
Past perfect continuous: I had been sleeping.
Future perfect continuous: I will have been sleeping.
Present simple: The car is repaired.
Present continuous: The car is being repaired.
Past simple: The car was repaired.
Past continuous: The car was being repaired.
Future simple: The car will be repaired.
Present perfect: The car has been repaired.
Past perfect: The car had been repaired.
Future perfect: The car will have been repaired.
As you have probably noticed, the passive continuous forms are possible only in the present continuous and past continuous.
2. Be + infinitive is used:
a) In impersonal orders or instructions.
No one is to wear a piercing at this school. (= No one is allowed to wear a piercing.)
She is to do it on her own. (= She must do it without help.)
b) To talk about plans.
The exhibition is to start on 21 June. (= They are planning to start the exhibition on 21 June.)
Paul was to marry Kate last year. (= They planned to get married last year.)
3. Was/were + infinitive is used if we want to say that something was certain and inevitable.
They fell in love with each other and didn't know that they were to break up soon.
He tried to climb the mountain again and again, but he was never to succeed.
4. Be about + infinitive indicates the immediate future.
Wait a minute, please. The film is about to end.
When I came to Sarah's place, she was just about to leave.