Features Of Auxiliary Verbs

There are four(4) main features which distinguish auxiliary verbs from other verbs. If a word or verb doesn't possess these features, it is not an auxiliary verb:

1.Negation

A word is an auxiliary verb if it can be used in negation.

Example

1. They are crying profusely.

             aux

They are not/aren't  crying profusely.

          aux.     aux not.

'Are' is an auxiliary verb because you can use it with the negator, 'not'. Once you can add 'not' to a word and it still remains grammatical,then the word is an auxiliary verb.  Try it with other auxiliary verbs you know and see, e.g. can, do, should, must etc.

'Not' is a negator adverb that you add to a positive preposition in order to change it to negative.

2. Interrogation

If a verb can be used in interrogative permutation, it means it is an auxiliary verb.

Examples

1. I can drive.            Can you drive?

2.I must drive.          Must I drive?

3. I should drive.      Should I drive? Etc.

The fact that the above statements can be turned to question, confirms the fact that 'can', 'must' & 'should' are auxiliary verbs.

Any verb that cannot be used in interrogative permutation cannot be an auxiliary verb.

Example

He happened to be there.

Happened he to be there?(ungrammatical)

In the above sentence, 'happened' is not an auxiliary verb because it cannot be used in interrogative permutation, so you add the auxiliary 'do' e.g. Did he happen to be there?(grammatical)

3. Code

It is a term used by J.R Firth. It is also called 'Avoidance of Repetition'. If a word or verb can be used as code, then it is an auxiliary verb.

Example

I can drive a car and so can my husband.

The above sentence is used instead of 'I can drive a car and my husband can a drive'. In order not to repeat the clause, 'my husband can a drive' we introduced the auxiliary, 'can' which functions as a code.

Anyone word  that cannot be used as a code is not an auxiliary verb.

Example
The audience laughed and so laughed the performer.

The above sentence is ungrammatical because 'laugh' is not an auxiliary verb and cannot  function as a code, so you add the auxiliary, 'do' to make it grammatical. Remember, 'do' is also an auxiliary verb.

'The audience laughed and so did the performer.'

4. Emphatic Affirmation

Auxiliary verbs generally are not stressed,e.g. he is writing a test. The auxiliary 'is' in this sentence is not stressed. 

When you want to emphasize something, you stress the auxiliary verb, e.g. You MUST write the test.

Ordinarily, you stress the lexical verb, e.g. You must WRITE the test.

In a nutshell, if a non lexical verb is stressed for purpose of emphasis, it is an auxiliary verb.

Note 

The auxiliary, 'do' is described as a dummy. It is used as an auxiliary only when a verbal group/phrase lacks one. It  functions as a lexical verb, e.g. I will do it very well. It chiefly functions in negation and interrogation, e.g. You do not ask/You did not ask. Do you like it? Did you ask?