English sentences must have at least one verb or verb phrase that is not simply an infinitive (to write), a gerund (writing), or a participle (written) without any helping verbs. Use helping (also called auxiliary) verbs with a main verb—in its base form or in a present participle or past participle form—to create verb phrases.

The most common auxiliaries are forms of be, have, and do. Have is used to form perfect tenses that indicate completed action (39e); be is used with progressive forms that show continuing action (39e) and to form the passive voice (39g).

The engineers have considered possible problems. [completed action]

The college is building a new dormitory. [continuing action]

The activists were warned to stay away. [passive voice]

As an auxiliary, do is used to show emphasis, to form questions, and to make negative statements.

I do respect my opponent’s viewpoint. [emphasis]

Do you know the answer? [question]

He does not like wearing a tie. [negative statement]

Arranging helping (auxiliary) verbs

Verb phrases can be built up out of a main verb and one or more auxiliaries.

Immigration figures rise every year.

Immigration figures are rising every year.

Immigration figures have risen every year.

Immigration figures have been rising every year.

Verb phrases have strict rules of order. The only permissible change to word order is to form a question, moving the first auxiliary to the beginning of the sentence: Have immigration figures been rising every year?

When two or more auxiliaries appear in a verb phrase, they must follow a particular order based on the type of auxiliary:

Very few sentences include all four kinds of auxiliaries.

Using modals

The modal auxiliaries—can, could, may, might, shall, should, will, would, must, and ought to—indicate future action, possibility, necessity, or obligation.

They will explain the procedure. [future action]

You can see three states from the top of the mountain. [possibility]

Students must manage their time wisely. [necessity]

They should examine the results of the study. [obligation]

No verb phrase can include more than one modal.

Shall was once used instead of will with the first person (I or we), but in U.S. English today, shall rarely appears except in legal documents and other very formal contexts. In earlier English, could, would, should, and might were used as past-tense forms. Today, could still functions to some extent as the past tense of can.

Ingrid can ski.

Ingrid could ski when she was five years old.

Modals for requests or instructions

Modals are often used in requests and instructions. If you use a modal such as could or would, you are politely acknowledging that the person you are talking to may be unable or unwilling to do what you ask.

Could you bring me a pillow?

Modals appearing in instructions usually indicate whether an action is suggested or required:

Modals to show doubt or certainty

Modals can also indicate how confident the writer is about his or her claims. Using may or might results in a tentative suggestion, while will indicates complete confidence:

The study might help explain the findings of previous research.

The study will help explain the findings of previous research.

Forming phrases with modals

Use the base form of a verb after a modal.

Alice can read Latin.

Sanjay should have studied for the test.

They must be going to a fine school.

In many other languages, modals such as can and must are followed by an infinitive (to + base form). In English, only the base form follows a modal.

Notice that a modal auxiliary never changes form to agree with the subject.

For the most part, modals refer to present or future time. When you want to use a modal to refer to the past, you follow the modal with a perfect form of the main verb (see 39e).

If you have a fever, you should see a doctor.

If you had a fever, you should have seen a doctor.

The modal must is a special case. The past tense of must is had to or needed to.

You must renew your visa by the end of this week.

You had to renew / You needed to renew your visa by last Friday.

Note, too, the different meanings of the negative forms must not and don’t have to.

You must not go to the party. [You are forbidden to go.]

You don’t have to go to the party. [You are not required to go, but you may.]