The modal verbs include can, must, may, might, will, would, should. They are used with other verbs to express ability, obligation, possibility, and so on. Below is a list showing the most useful modals and their common meanings:
Modal verbs are unlike other verbs. They do not change their form (spelling) and they have no infinitive or participle (past/present). The modals must and can need substitute verbs to express obligation (have to) or ability (able to) in the different tenses. Here are some examples:
Modals are auxiliary verbs. They do not need an additional auxiliary in negatives or questions. For example: Must I come? ( Do I must come? ), or: He shouldn't smoke ( He doesn't should smoke ).
Important: The explanations and examples on this page are just an introduction to this extensive and complex area of English grammar. Elsewhere on this site you can read more about each of the modal verbs and their difficulties.
Students of English who want to explore the topic further are recommended to consult a good reference work, such as Swan's Practical English Usage.
There are several interactive quizzes on the modals in the Verb grammar: Miscellaneous drop-down menu on the Grammar index.