Modal verbs are a kind of auxiliary verb. They facilitate the main verb for suggesting potential, expectation, permission, ability, possibility, and obligation.
When used with the main verb, modal verbs do not end with –s for the third-person singular. Modal auxiliary verbs never change form, but they have a different form for past tense.
The modal auxiliaries include:
| Present Tense | Past Tense |
| WillCanMust (have to)MayShould (ought to) (had better) | Would (used to)Could(Had to)MightShould (ought to) |
NB: The words in parentheses ( ) are semi-modals. They have the same meaning, but they are different grammatically.