Modal verbs are auxiliary verbs.
They do not expression action, they have no conjugation and they depend on a main verb that is always in infinitive form. No preposition is necessary between verbs. The eight classic modal verbs are: can, could, may, might, should, would, will, and must.
The structure to use modal verbs is:
(Subject) + Modal + (main verb infinitive) + (object)
I can speak English
I must study every day
I should practice more
(Subject) + Modal + not + (main verb infinitive) + (object)
I shouldn't worry so much
I won't forget this
I wouldn't go if I were you
Modal + (subject) + (main verb infinitive) + object
Can't you see?
Must I go?
Could you pass the salt, please?
Each modal verb is used in a different context and can express different messages:
The modal verb 'can' is mostly used to express ability, permission or request.
I can drive a car, but I can't pilot an airplane
I can enter, but I don't want to
Can you pass me the salt? Can you help me?
'Could' is used to request or to express past ability, usually implicit that the action can't be performed today.
I could sleep all day when I was a child
I couldn't drive a car when I was 14
Could you call me an Uber? Could you walk my dog?
'May' is used to either ask for permission in a more formal/polite way or to express a considerable possibility.
I may be asleep at 11pm, call earlier
I may not visit my family this weekend if it snows
May I help you? May I take your order?
'Might' is used to express remote possibility.
You might see the Aurora Borealis during the summer, but it's very rare.
It might rain later.
Might it hurt?
'Should' is used for advising and recommending.
You should drink water every day.
You shouldn't eat a lot of sugar.
Should I be worried?
'Would' is used to express hypothetical action.
If I had a million dollars, I would buy a beautiful chocolate cake!
I wouldn't do that if I were you.
Would you do me a favor?
'Will' is the modal verb used to express future in English.
I will visit Japan.
I won't stop studying.
Will you go to the show?
'Must' is used to express 'obligation' or 'prohibition' depending if the clause is affirmative or negative.
You must drink water every day
You must not mistreat other people
Must I go?