Modal Verbs–Definition and Usage

Modal verbs are auxiliary verbs (also called helping verbs) like can, will, could, shall, must, would, might, and should. After a modal verb, the root form of a verb is generally used. The word to should not appear after a modal verb. An exception is the phrase ought to, which is considered a modal verb.

Modal verbs add meaning to the main verb in a sentence by expressing possibility, ability, permission, or obligation.

You must turn in your assignment on time.

He might be the love of my life.

The doctor can see you now.

When a modal verb precedes the main verb in a sentence or clause, use the root of the verb rather than the infinitive, which contains the word to.

The doctor can to see you now.

The one case in which we deviate from this rule and use the full infinitive form of a verb is with the modal phrase “ought to”

The doctor ought to see you now. (You’ve waited long enough.)

To Do as a Modal Verb

The verb to do can be used as a modal verb or a main verb. In this sentence, to do is being used as a main verb and can is being used as a modal verb.

You can do it.

Whereas in this example, do is used as a modal verb.

You do know how to sing!

Remember, however, that do can be used as a modal verb only if there is no other modal verb present in the sentence.

Thank you, I can do sing.

Thank you, I do sing.

Thank you, I can sing.