Simple Definition:

Modals, in English grammar. are a type of auxiliary verbs [verbs that appear together with other verbs and add to their meaning] that express different meanings such as ability, possibility, necessity, permission, and expectation. They are used with the base form of a verb to modify the main verb in a sentence.

Modals include words like "can," "could," "may," "might," "must," "shall," "should," "will," "would," and "ought to."

For example:

• I can swim. (ability)
• She may come to the party. (possibility)
• You should study for the exam. (necessity)
• They must not enter without permission. (prohibition)

Very Simple Definition:

Modals in English grammar are special words that help us talk about what is possible, necessary, or allowed.

For example:

• I can swim. (shows the ability to swim)
• She may come to the party. (shows the possibility of coming)
• You should study for the exam. (It’s a good idea to study)
• They must not enter without permission. (They are not allowed to enter)