“All the auxiliary verbs except be, do and have are called modals. Unlike other auxiliary verbs, modals only exist in their helping form. [Besides,] they cannot act alone as the main verb in a sentence.”
The modal verbs are: can, could, may, might, must, should, will, shall and would. There are other verbs which behave like modal verbs, such as be able to, have to, need to and had better. We use them to talk about ability, necessity, possibility, obligation, prohibition, permission, requests, offers and suggestions.
1. Most modal verbs cannot be used in past/future tenses.
2. When used in third person, present tense modals do not have '-s' ending.
3. Use "not" to make modals negative.