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What are Auxiliary Verbs

Sometimes, a sentence has two verbs. 

He needs to stop.

She misses playing tennis.

I want to drive my car.

We practice singing.

The first verb is the auxiliary verb, and the second verb is the main verb.

Change Auxiliary, Leave Main

Only change the auxiliary verb. Leave the main verb alone. 

1) If the subject is He/She/It, add "s" to the auxiliary verb.

He has to finish his dinner.

She wants to come to the party.

2) If past, make auxiliary verb past tense but leave main verb in present simple. 

You helped me do my homework.

I pretended to sleep.

Do I use the word "to"? 

That depends on the auxiliary verb. 

I can't wait to see you.

Some auxiliary verbs are followed by "to".

She started to cry.
She started crying.

Some auxiliary verbs are followed by either "to" or "verb-ing".

He asked me to visit.

Some auxiliary verbs are followed by an object and "to".

We ended up going home.

Some auxiliary verbs are followed by a verb-ing. 

They can't swim.

Some auxiliary verbs are followed by a verb (present simple). 

They made me buy it.

Some auxiliary verbs are followed by an object and no"to".

A very special auxiliary: Modal Verbs

Modal verbs are auxiliary verbs that follow some special rules.

1) Modal verbs are not followed by "to" or "verb-ing". They are followed by a verb in the present simple.

It might rain.

He won't admit it.

2) Modal verbs do not take "s" when the subject is he/she/it.

She must be excited.

He may leave. 

3) Modal verbs are irregular in past tense. 

They couldn't speak Spanish.

She might not have gotten it.

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