A verb is a word which can assert something (usually an action) concerning a person, place or thing.
The wind blows.
The horses ran.
The fire blazed.
Her jewels sparkled.
Tom climbed a tree.
The dynamite exploded.
Some verbs express state or condition rather than action.
The treaty still exists.
The book lies on the table.
Near the church stood an elm.
My aunt suffers much from headache.
A group of words may be needed, instead of a single verb, to make an assertion.
A group of words that is used as a verb is called a verb-phrase.
You will see.
The tree has fallen.
We might have invited her.
Our driver has been discharged.
Certain verbs, when used to make verb-phrases, are called auxiliary (that is aiding) verbs, because they help other verbs to express action or state of some particular kind.
Thus, in “You will see," the auxiliary verb WILL helps see to express future action.
In “We might have invited her," the auxiliaries MIGHT and HAVE help invited to express action that was possible in past time.
The auxiliary verbs are….is (are, was, were, etc.), may, can, must, might, shall, will, could, would, should, have, had, do, did. Their forms and uses will be studied in connection with the inflection of verbs.
The auxiliary verb regularly comes first in a verb-phrase and may be separated from the rest of it by some other word or words.
Where was Washington born?
The boat was slowly but steadily approaching.
Is (in its various forms) and several other verbs may be used to frame sentences in which some word or words in the predicate describe or define the subject.
1. Gold is a metal.
2. Charles is my friend’s name.
3. The colors of this butterfly are brilliant.
4. Iron becomes red in the fire.
5. Our condition seemed desperate.
6. Bertram proved a good friend in this emergency.
7. My soul grows sad with troubles. - Shakspere.
In the first sentence, the verb is not only makes an assertion, but it also connects the rest of the predicate (a metal) with the subject (gold) in such a way that a metal serves as a description or definition of gold.
In sentences 4–7, becomes, seemed, proved and grows are similarly used.
In such sentences is and other verbs that are used for the same purpose are called copulative (that is, “joining") verbs.
IS in this use is often called the copula, that is, the “joiner" or “link."
The forms of the verb is are very irregular. Among the commonest are : am, is, are, was, were and the verb-phrases has been, have been, had been, shall be, will be.