Modal

Some features of the modals:

Modal auxiliaries have both present and a past tense form, except for must and need:

You might argue that need has a past tense form needed, but this is only true for the lexical verb. When need is used as an auxiliary, it can't come up in a past tense form. The difference is demonstrated by the next two examples:

In the first example the verb is used as an auxiliary. Putting the auxilliary need in a past tense form yields an ungrammatical sentence. In the second example, however, it is used as a lexical verb. Here the past tense seems to do just fine.

The form of the modal auxiliary:

Since a modal is always the first auxiliary in a predicate, it follows that the modal is always tensed. After all, the first verb in a predicate is always responsible for carrying tense. Therefore, whenever a modal is present, it is the first verb in the predicate and it is tensed. Modal auxiliaries are exceptional when it comes to subject-verb agreement. Modals are the only types of verb that do not have subject-verb agreement. That means that they don't get the -s ending for the third person singular:

The form of the following verb:

The verb following a modal auxiliary always appears in its basic stem form. Compare the example sentences already given on the Auxiliaries page and notice the verbs following the modals in their non-finite form: