The verbs be (is, am, are, was and were), have and do are called auxiliary verbs when they are used with ordinary verbs to make tenses, passive forms, questions and negatives.
The verbs can, could, may, might, will, would, shall, should, must and ought are called modal verbs. Modal verbs are used before ordinary verbs and express ideas such as permission, possibility, certainty and necessity. Need and dare are also sometimes used like modal verbs.
The auxiliary verb be is used:
1) in the formation of the continuous tenses
- I am writing.
- She was singing.
- It was raining.
- They were playing.
2) in the formation of passive verb forms
- I was shocked.
- The boys were praised.
- He was questioned.
Be can be followed by an infinitive. This structure is used to talk about an arrangement, a plan, an agreement or a command.
- I am to see him tomorrow. (Arrangement)
- They are to be married next month.
- You are to leave at once.
Auxiliary verb have
The auxiliary have has the following uses:
1) in the formation of the perfect tenses
- She has come.
- They have returned.
- She has been waiting.
2) with the infinitive to indicate obligation
- I have to be there by 12 o’clock.
- She has to finish the work.
Had + infinitive can be used to talk about an obligation that existed in the past.
- I had to be there by 12 o’clock.
In questions and negatives, have to and had to are used with do, does and did.
- She had to go.
- Does she have to go?
- She doesn’t have to go.
- They have to go.
- Do they have to go?
- They don’t have to go.
- I had to go.
- Did I have to go?
- I didn’t have to go.