Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

Verbs
A closer look at predicates
Introduction to sentence structure

Now let’s look more closely at the part of the sentence that makes a statement about the subject—the predicate.

Verbs

As noted earlier, verbs either show the action—whether visible or invisible—or the condition of a subject. Those that show the condition of the subject do so by linking the subject to a complement that follows the verb.

Another important fact is that verbs change in form to communicate changes in time. These various forms are called a verb’s tenses. Sometimes forming a verb tense involves nothing more than the addition of an ending; for example, adding a -d or -ed ending can form the past tense of a regular verb. But other times it involves the addition of a helping verb, which is simply a verb that helps another verb form a particular tense or a mood. Helping verbs include forms of the verb to be such as is, are, was, were, and will be. They also include forms of the verb to have, such as has, have, and had.

Other helping verbs are the modal auxiliaries: can, could, may, might, shall, should, must, will, and would.

There are also helping verbs that give extra emphasis to the predicate: do, does, and did.

When a verb joins up with a helping verb, it forms a verb phrase. For example: is living, will be reviewed, has answered, could remember, might sing, and did pay. Other verb phrases contain more than one helper. Examples are: will have been dedicated, should be invited, and may have promised.