Modal verbs express the mood of verbs, permission, ability, obligation or prediction depending on the context.

Modal auxiliaries have no meaning unless accompanied by a verb in the infinitive without "to".

Two modal auxiliaries are cannot be combined:
e.g. "I will can go" or "He must will work" are impossible sentences.

AUXILIARY	VERB FORMS	USES
can	I can speak Portuguese.	ability
	Can you pass me the salt?	permission
	He can't have done it by himself.	impossibility
	She can be hard to understand.	possibility
	He will be able to speak english.	"will be able" future for can
could	I could walk for miles when he was young.	past of can
	Could you help me please?	conditional of can
may	He may come to the cinema.	possibility
	May I come in?	permission in questions
might	He might come tonight.	possibility (less than may)
will	He will be coming in the evening.	future
shall	Shall we go?	suggestions
would	They would come if they had a car.	conditional
	He would have told him if he knew the truth.	conditional past
should	You should stop smoking.	advice
	I should call my mother more often.	moral obligation
	They should be here by now.	probability
must	I must work harder.	obligation
	She must be in her thirties.	strong probability
	I had to have a drink last night.	"have to" past for must
	I will have to finish the report tomorrow.	"will have to" future must

exercise