As a principal verb

Be and its forms can be used as principal verbs. They are then called linking verbs because they link the subject with a following word.

Susie is my teacher. (Here the principal verb is links the subject Susie with the noun teacher.)
She is very kind. (Here the principal verb is links the subject she with the adjective kind.)
The car was in the garage. (Here was links the subject with an adverbial phrase.)
They are very efficient. (Here are links the subject they with the adjective efficient.)

Be can be used as a full verb by itself in the sense of exist.

God is. (= God exists.)
I think, therefore I am. (=I think, therefore I exist.)

Be can be used to express a command or request.

Be quiet.
Don’t be silly.
Be a good girl.

Be as an auxiliary verb

Be can be used as an auxiliary verb. Read the sentences given below:

I am writing.
She is sleeping.
We are coming.
They were waiting.
He was working.

In the example sentences given above, the different forms of be (is, am, are, was, were) combine with the present participles of verbs to form the present and past continuous tenses.

I was being victimized.
You are being considered for the job.
The dinner is being prepared.
They were being questioned.

Here the forms of be combine with the passive of the present participle to form the passive of the continuous tenses.

I was sent to the market.
He was punished for stealing.
They were treated badly.
You are invited.

Here the forms of be combine with the past participles of transitive verbs to form the passive voice.

He is gone.
You are grown up.
They were engaged in a heated argument.

Here the forms of be combine with the past participles of certain intransitive verbs. Note that the sentence He is gone means the same as the sentence He has gone. Similarly, You are grown up means the same as You have grown up.

Uses of Have

As a principal verb

Have can be used as a principal verb in the sense of possess, take, experience, receive etc.

They have a car. (Possess)
I have two kids. (Possess)
We had a strange experience. (Experience)
We have dinner at 8 pm. (Take)
I had a letter from my mother. (Receive)

When have is used as a principal verb, it connects the subject with a following noun.

Have as an auxiliary verb

As an auxiliary verb have combines with the past participle to form the present and past perfect tenses.

I have finished writing.
She has created a problem.
You have turned down my offer.
I have decided to quit.
I have heard of this before.
She had left before I arrived.
They had forgotten to post the letter.