English auxiliary verbs exhibit four properties. They are:

1. Auxiliaries alone can be negated by adding not.

She can swim. (Affirmative)
She cannot swim. OR She can’t swim. (Negative)
They have come. (Affirmative)
They have not come. OR They haven’t come. (Negative)
She would smoke. (Affirmative)
She would not smoke. OR She wouldn’t smoke. (Negative)

Compare this property with other verbs.

She smokes. (Affirmative)
She does not smoke. (Negative) (NOT She smokes not.)
He came. (Affirmative)
He did not come. (Negative) (NOT He came not.)

2. Auxiliaries alone can be inverted.

She is working. (Affirmative)
Is she working? (Interrogative)
They have arrived. (Affirmative)
Have they arrived? (Interrogative)

Compare this property with other verbs.

He smokes. (Affirmative)
Does he smoke? (Interrogative) (NOT Smokes he?)

3. Auxiliaries alone have the ability to allow a following verb phrase to be deleted.

Will she accept the job? I think she will (accept the job). Here the verb phrase ‘accept the job’ isn’t necessary, though it can be used.

Have they come? They have (come). Here the verb come is not necessary though it can be used.

4. Auxiliaries alone can be emphasized.

She CAN go.
You MUST wait.
She DOES smoke.

In all the three sentences given above, the stress falls on the auxiliaries.