Modal verbs are also called modal auxiliaries or modals. Modal verbs are sometimes called defective verbs, because they do not have all the functions of main verbs or auxiliary verbs. They can't be used without a main verb, can't form gerunds or participles, and do not have any endings to show person, number, or tense. Modal verbs form questions without the help of the other auxiliary verbs. Some modal verbs are rarely used in questions. Modal verbs also have quite a few peculiarities in the formation of tenses.

Grammatical peculiarities