the main verb

The Main Verb

Sometimes there is more than one kind of verb in a sentence. There are auxiliary verbs , modal verbs, and main verbs (sometimes called full or non-auxiliary verbs).
The main verb expresses the main action or state of being of the subject in the sentence and changes form according to the subject (singular, plural, 1st person, 2nd person, 3rd person).
Most statements in speech and writing have a main verb.
The main verb changes its form according to the verb form (perfect tense, past tense, simple tense etc).
For example:

• Dogs usually chase cats.
• But my cat chases my dog.
• My cat is chasing my dog.
• My dog has sometimes chased my cat.
• But, only because my cat ate my dog's dinner.
• My cat has been eating my dog's dinner a lot.

Auxiliary Verbs

Auxiliary Verbs are the verbs be, do, have, will when they are followed by another verb (the full verb) in order to form a question, a negative sentence, a compound tense or the passive.

The verb "be"

The verb be can be used as an auxiliary and a full verb. As an auxiliary we use this verb for compound tenses and the passive voice. Note that be is an irregular verb:

Simple Present:

I am, he/she/it is, we/you/they are

Simple Past:

I/he/she/it was, we/you/they were

Past Participle:

been

You can tell that in the following sentences be is an auxiliary because it is followed by another verb (the full verb). (For progressive forms use the "-ing" form of the full verb; for passive voice, use the past participle of the full verb.)
Progressive Forms

Present Progressive:

He is playing football.

Past Progressive:

He was playing football.

Present Perfect Progressive:

He has been playing football.

Past Perfect Progressive:

He had been playing football.

Passive

Simple Present/Past:

The house is/was built.

Present/Past Perfect:

The house has/had been built.

Future I:

The house will be built.

"be" as a full verb
The verb be can also be a full verb. In this case, it's not followed by another verb. If be is used as a full verb, we do not need an auxiliary in negative sentences or questions.

positive sentence:

They are fifteen years old.

negative sentence:

They are not fifteen years old.

question:

Are they fifteen years old?

The verb "have"

The verb have, too, can be used both as an auxiliary and as a full verb. As an auxiliary we use this verb to form compound tenses in active and passive voice. (Use the past participle of the full verb.)
Compound Tenses - Active Voice

Present Perfect Simple:

He has played football.

Past Perfect Simple:

He had played football.

Present Perfect Progressive:

He has been playing football.

Past Perfect Progressive:

He had been playing football.

Compound Tenses - Passive Voice

Present/Past Perfect:

The house has/had been built.

Note that have is an irregular verb, too:

Simple Present:

I/we/you/they have, he/she/it has

Simple Past:

I/he/she/it/we/you/they had

Past Participle:

had

"have" in positive sentences
As a full verb have indicates possession. In British English, however, we usually use have got (have being the auxiliary, got the full verb).

full verb:

I have a car.

auxiliary verb:

I have got a car.

"have" in negative sentences and questions
When we use have as a full verb, we must use the auxiliary do in negative sentences and questions. If we use have got, however, we do not need another auxiliary.

have as a full verb:

I do not have a car.

Do I have a car?

have as an auxiliary verb:

I have not got a car.

Have I got a car?

The verb "will"

The verb will can only be used as an auxiliary. We use it to form the future tenses.
The auxiliary verb "will"

Future I:

He will not play football.

Future II:

He will have played football.

The verb will remains the same for all forms (no "s" for 3rd person singular). The short form for negative sentences is won't.'

Examples:

I will, he will

I will not = I won't

The verb "do"

The verb do can be both an auxiliary and a full verb. As an auxiliary we use do in negative sentences and questions for most verbs (except not for be, will, have got and modal verbs) in Simple Present and Simple Past. (Use the infinitive of the full verb.)
The auxiliary "do" in negative sentences

Simple Present:

He does not play football.

Simple Past:

He did not play football.

The auxiliary "do" in questions

Simple Present:

Does he play football?

Simple Past:

Did he play football?

The verb do is irregular:

Simple Present:

I/we/you/they do, he/she/it does

Simple Past:

I/he/she/it/we/you/they did

The full verb "do"
As a full verb we use do in certain expressions. If we want to form negative sentences or questions using do as a full verb, we need another do as an auxiliary.

positive sentence:

She does her homework every day.

negative sentence:

She doesn't do her homework every day.

question:

Does she do her homework every day?

Sentences without the auxiliary "do"

In the following cases, the auxiliary do is not used in negative sentences/questions:
the full verb is "be"

Example:

I am not angry. / Are you okay?

the sentence already contains another auxiliary (e.g. have, be, will)

Example:

They are not sleeping. / Have you heard that?

the sentence contains a modal verb (can, may, must, need, ought to, shall, should)

Example:

We need not wait. / Can you repeat that, please?

the question asks for the subject of the sentence

Example:

Who sings that song?

Auxiliary Verbs

Auxiliary verbs are used together with a main verb to give grammatical information and therefore add extra meaning to a sentence. Information that is not given by the main verb.
They are used to form the passive voice.
They are used to form the continuous tense.
They are used to form the perfect tense.
Be, Do and Have are auxiliary verbs, they are irregular verbs and can be used as main verbs. The verbs 'to be' and 'to have' are the most commonly used auxiliary verbs and work alongside the main verbs in any statement.
Modal verbs are also auxiliary verbs, but will be treated separately, these are can, could, may, might, must, shall, should, will, and would, they differ from the others in that they can never function as a main verb.

To be

Be is the most common verb in the English language. It can be used as an auxiliary and a main verb. It is used a lot in its other forms.
Base form = be
Present form = am/is/are
Past form = was/were
Present Participle / Gerund = being
Past Participle = been

To do

The verb do is one of the most common verbs in English. It can be used as an auxiliary and a main verb. It is often used in questions.

Base form = do
Present form = do/does
Past form = did
Present Participle / Gerund = doing
Past Participle = done

!Note - The auxiliary verb 'do' is always followed by the base form (infinitive).

To have

Have is one of the most common verbs in the English language.
Base form = have
Present form = have / has
Past form = had
Present Participle / Gerund = having
Past Participle = had

Modal Verbs

All the auxiliary verbs except be, do and have are called modals. Unlike other auxiliary verbs modals only exist in their helping form; they cannot act alone as the main verb in a sentence.

Be, do, and have also differ from the other auxiliaries in that they can also serve as ordinary verbs in a given sentence.

The modal verbs are:-

CAN / COULD / MAY / MIGHT / MUST / SHALL / SHOULD / OUGHT TO / WILL / WOULD

Modal	Example	Uses
Can	They can control their own budgets. We can’t fix it. Can I smoke here? Can you help me?	Ability / Possibility Inability / Impossibility Asking for permission Request
Could	Could I borrow your dictionary? Could you say it again more slowly? We could try to fix it ourselves. I think we could have another Gulf War. He gave up his old job so he could work for us.	Asking for permission. Request Suggestion Future possibility Ability in the past
May	May I have another cup of coffee? China may become a major economic power.	Asking for permission Future possibility
Might	We'd better phone tomorrow, they might be eating their dinner now. They might give us a 10% discount.	Present possibility Future possibility
Must	We must say good-bye now. They mustn’t disrupt the work more than necessary.	Necessity / Obligation Prohibition
Ought to	We ought to employ a professional writer.	Saying what’s right or correct
Shall (More common in the UK than the US)	Shall I help you with your luggage? Shall we say 2.30 then? Shall I do that or will you?	Offer Suggestion Asking what to do
Should	We should sort out this problem at once. I think we should check everything again. Profits should increase next year.	Saying what’s right or correct Recommending action Uncertain prediction
Will	I can’t see any taxis so I’ll walk. I'll do that for you if you like. I’ll get back to you first thing on Monday. Profits will increase next year.	Instant decisions Offer Promise Certain prediction
Would	Would you mind if I brought a colleague with me? Would you pass the salt please? Would you mind waiting a moment? "Would three o`clock suit you?" - "That’d be fine." Would you like to play golf this Friday? "Would you prefer tea or coffee?" - "I’d like tea please."	Asking for permission Request Request Making arrangements Invitation Preferences

!Note The modal auxiliary verbs are always followed by the base form.

The verb used to, which is explained here, can also be used like a modal verb.