Auxiliaries are structure-class words, so should not be confused with form-class words. Auxiliary verbs do not have most of the characteristics of true verbs. Instead, as part of the Main Verb Phrase Formula, auxiliary verbs function with main verbs to do things like:

Alter meaning:

• Students pass this test.
• Students might pass this test.

Express different tenses:

• A bright green bird nested there.
• A bright green bird has nested there.
• A bright green bird had nested  there.
• We birdwatch.
• We are birdwatching.
• We were birdwatching.

Create questions:

• He leaves tomorrow.
• Does he leave tomorrow?
• Is he leaving tomorrow?

There are three forms of auxiliary verb:

1) Modals

Modals include

• can
• could
• will
• would
• shall
• should
• may
• might
• must

The nine modals always precede the main verb as well as any other auxiliaries in the Main Verb Phrase. 

Modal auxiliaries do not have many of the characteristics of true verbs. They cannot take tense morphemes or be made into commands or negatives.

No tense morphemes (present-tense, past-tense, past-participle, present-participle):

• *She coulds run a marathon.
• *I mighted have eaten.
• *You shoulden walk alone at night.

No commands:

• *Would eat your breakfast!
• *Shall go to bed!

No negatives:

• She cannot eat. In this case, not negates the main verb eat. Without that verb, you don’t know what she cannot do.
• You may not run with scissors. In this case, not negates the main verb run. Without that verb, you don’t know what you may not do.

Modal auxiliaries signal specific meanings by modifying the phrase or sentence’s main verb. For example, they can indicate:

Probability

• Students pass this test.
• Students might pass this test.

Condition

• I swam when I was younger.
• I could swim when I was younger.

Obligation

• You have car insurance.
• You must have car insurance.

Possibility

• I eat breakfast.
• I may eat breakfast.
• I will eat breakfast.

2) Have and Be

HAVE and BE are the most common auxiliaries, and they will each inflect on the verb that follows them in the Main Verb Phrase, but they can also be difficult to analyze because they can also serve as Main Verbs, which would identify them in those contexts as form-class words. Be sure that you are able to distinguish between HAVE or BE as an auxiliary and HAVE or BE as a main verb.

The auxiliary have occurs before the past participle [-en] of a main verb in order to form the perfect tense:

Present-Perfect

• A bright green bird has nested there.
• The judges have voted.

Past-Perfect

• A bright green bird had nested there.
• The judges had voted.

Caution: Have can function as a main verb as well as an auxiliary:

• I have chicken pox.
• She has all the latest gadgets.
• I had my favorite meal for my birthday.
• The Smiths had a baby.

If you analyze sentences like these, you must designate HAVE as a main verb, not an auxiliary. HAVE is only an auxiliary when it precedes another verb (or a BE auxiliary) to create a perfect form.

The auxiliary be occurs before the present participle [-ing] of a main verb in order to form the progressive tense:

Present-Progressive

• Harvey is cooking squash.

Past-Progressive

• Harvey was cooking squash.

As the only exception to the Main Verb Phrase Formula, the auxiliary be can also occur before the past participle [-en] of a main verb in order to form the passive:

Present Passive

• The National Anthem is sung by the whole crowd.

Past Passive

• The National Anthem was sung by the whole crowd.

Caution: Be can act as a main verb as well as an auxiliary:

• Keoni is a student.
• The lawyers were all in court.

If you analyze sentences like these, you must designate BE as a main verb, not an auxiliary. BE is only an auxiliary when it precedes a main verb to create either the progressive form or the passive.

3) Do

DO gets a separate category because DO acts like a modal in that it does not inflect on the main verb, but DO can also serve as a Main Verb.

The auxiliary do occurs in a number of grammatical processes, including:

Creating Questions:

• He leaves tomorrow.
• Does he leave tomorrow?

Forming Negatives:

• He wants curry for dinner.
• He doesn’t want curry for dinner.

Placing Emphasis:

• This tastes delicious.
• This does taste delicious!

Substitution:

• We want chicken curry more than they want chicken curry.
• We want chicken curry more than they do.

Caution: Do can also act as a main verb:

• Shawna did her own make-up.
• That meal did me a world of good.
• You will do as you’re told.

If you analyze sentences like these, you must designate DO as a main verb, not an auxiliary.

To test your understanding of the concepts discussed on this page, begin with the link below for an example practice exercise:

AUXILIARY SAMPLE EXERCISE

For a bit more of a challenge, analyze the following sentence from Robinson Crusoe for auxiliaries.

I told my mother that my thoughts were bent upon seeing the world that I should never settle to anything with resolution enough to go through with it, and my father should be giving me his consent than force me to go without it.

To review your answers to these two samples, check the AUXILIARY SAMPLES ANALYSES page.