Modal verbs are a kind of auxiliary verb used to add functionality to other verbs, often to show levels of certainty, ability, possibility, or obligation.
Examples of modal verbs include can, could, may, might, shall, should, will, would, and must.
Like other auxiliary verbs, modal verbs are formed by placing them before other verbs.
subject + modal verb + infinitive (dictionary form of verb)
Modal verbs are different from most other verbs in their formation. Modals verbs come before infinitives, but the to is dropped before the infinitive.
ex. You can play the piano. (NOT You can to play the piano.)
Modal verbs do not have ~s on the third person singular (he/she/it).
ex. She can swim. (NOT She cans swim.)
Questions and negatives do not use do with modals verbs.
ex. Can you speak Russian? (NOT, Do you can speak Russian?)
There is no infinitive or past participle form of modals.