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