English Grammar Drills - Mark Lester 2009

Modal Auxiliary Verbs
8 Basic Verb Forms
Part 2 Verb Phrases

There are five modal auxiliary verbs: can, may, must, shall, and will. These verbs have a unique history. They have no base form, no infinitive form, no present participle form, and no past participle form. The modal auxiliary verbs can only be used in the present and past-tense forms (though as we will see in the next chapter, their present and past forms almost never actually mean present or past time). Here is the complete list of forms:

Present tense     Past tense

can       could

may       might

must       —

shall       should

will       would

Notice that the present-tense form must has no corresponding past-tense form. This is the only instance in English where there is a present-tense form with no equivalent past-tense form.

The modal auxiliary verbs are also unique in that they do not add an -s in the third-person singular form. For example:

Correct third-person singular   Incorrect third-person singular

He can go.      X He cans go.

He may go.      X He mays go.

He must go.     X He musts go.

He shall go.      X He shalls go.

He will go.      X He wills go.

The reason for this odd exception to the normal rule is historical. All of the present-tense modal auxiliary verbs are actually derived from past-tense forms, and so they cannot be used with a present-tense ending.