Modals are auxiliary verbs that express necessity, possibility, permission, or ability. Here are the main modals and their common uses:
Can/Could -- Ability:
"She can swim.
"Permission:
"Can I leave early?"
Possibility:
"It could rain tomorrow.
"Polite request:
"Could you help me?"

May/Might

Permission:
"May I use your phone?
"Possibility: "
It might snow tonight.

"Must

Obligation:

"You must wear a seatbelt.
"Deduction:
"He must be at work now.

"Shall/Should
Suggestion: "Shall we go?"
Advice: "You should see a doctor.

"Will/Would
Future intention: "I will call you tomorrow.
"Polite request: "Would you mind closing the door?
"Habitual action: "She would always bring us cookies.

"Ought "
toAdvice:
"You ought to apologize.

"Modals don't change form based on the subject and are followed by the base form of the main verb.