USE They are used to talk about, for example,possibility,willingness,abilityobligation,certainty and permission. FORM -They are always followed by the infinitive without to,except ought . - They take the same form in all persons. -They don´t use auxiliary verbs to form the interrogative and negative. -We sometimes use expressions such as BE ABLE TO , BE ALLOWED TO or HAVE TO, instead of modal verbs for certain meanings and forms which are not possible with modal verbs. - When we talk about the past, we can use modal verb+ have+participle. We use this structure, for example, to talk about things that possibly happened or things that did not happen.
We use CAN to talk about ability. The negative is CANNOT( can´t) Can you swim? He can play the guitar. I can´t open this bottle. We can use be able to instead of can : Are you able to swim?
We use COULD to say that someone had the general ability to do something in the past. I could swim when I was 4 years old. WAS or WERE ABLE TO is also used with this meaning. I was able to swim when I was 4 years old. But when we want to say that someone had the ability to do something,and that they did it in a particular situation, we must use WAS or WERE ABLE TO ( could is not possible) Even though I´d hurt my leg, I was able to swim back to the boat ( managed to or succeeded in+ ing) There is an exception with the verbs of perception: see, hear,smell,taste,feel and some verbs of thinking eg understand, remember.We use COULD with these verbs when we actually did these things in particular situations. I could hear a noise outside my bedroom door. We use COULD NOT ( COULDN´T) for both general ability and particular situations. My grandmother couln´t dance. He tried very hard,but he couldn´t swim back to the boat.
Can I borrow your dictionary? Could I borrow your dictionary? (less direct,more polite than can) May I borrow your dictionary? ( more formal, more "correct") but can and could are more common. Might I use your dictionary? (less direct, more formal style)
To give permission, we use CAN or MAY ( but not could or might) Can I use your pen for a moment? Yes, of course you can. Could I make a suggestion? " Of course you may"
When we talk about things that are already permitted or not permitted ( eg when there is a law or a rule), we use can ('t) or be (not) allowed to. You can' t smoke / aren't allowed to smoke in this room.
We use COULD HAVE + PAST PARTICIPLE to say that someone had the ability or the opportunity to do something in the past but did not do it.
We use both MUST and HAVE TO to express obligation or necessity, but there is sometimes a difference between them. MUST---- The authority comes from the speaker. You must drive carefully( I insist) HAVE TO.. t