Can/ Be able to / Can’t CANDifferent uses: Abilities or capacities (to know or to be able to). Mary can swim very fast.Request, ask or give permissionCan you call me tonight.PossibilityI can meet you later. It can also be used for suggestions. You can eat ravioli if you like pasta.
Be able toIt expresses abilities like can and it is used in all the verbal tenses where can is not used. I was able to finish my homework on timeCan’tDifferent uses: Impossibility in the presentMary can’t swim very fastLack of ability (not to know) or capacity (not to be able to): I can’t eat a whole cake by myselfProhibitionYou can’t drive without a licenceDisbeliefThat can’t be the price – it’s much too cheap.
Could It is the past of can and it is used to express: Ability or capacity in the pastShe could run fast when she was a childPolite requestCould you help me with these suitcases?Polite suggestionYou could exercise and eat healthier foodPossibility –less probable than with can-Mark could join us the cinema.
May/ might Both of them express possibility, but might is more remote. It may/ might rain tomorrow In questions, may is the polite way of asking for things. May I have a coffee, please?
Would In questions, it is a formal way of asking for things. Would you open the window, please?With the verb “like” is used to make offers and invitations. Would you like something to drink?
Must / Have to Both express obligation, but must is only used in the present and have to in the other tenses. Authority people use must, while have to is used by everybody. You must bring your books to classI have to buy the tickets today. Must is also used to express a logical deduction about present fact. She’s got a great job. She must be very happy.
Need to / Needn’t Need to is not a modal, but it is used in affirmative sentences, like have to, to express obligation and necessity. I need to cook dinner tonight. Needn’t, on the contrary, is a modal and indicates lack of oblication and necessity, like don’t have toYou needn’t bring anything to the party.
Musn’t / Don’t have to Musn’t shows prohibition. You musn’t exceed the speed limitDon’t have to means not have to, i.e., lack of obligation and necessity, like needn’tI don’t have to get up early tomorrow
Should /Ought to Both of them express advise or opinion, but should is used more frequently, since ought to is quite strange in negative and interrogative. You should/ought to improve your pronunciation
Shall It is used in the interrogative to offer oneself to do something and to make a suggestion. Shall I help you with your luggage?
MODAL PERFECTS Must have + participleIt expresses a logical conclusion about a past fact. Rob has arrived late. He must have been in a traffic jam. May/might have + participleWe use it to make a supposition about something in the past. She may/might have taken the wrong bus.
Could have + participleAbility to do something in the past which in the end was not doneYou could have asked the doctor before taking the medicine. Couldn’t have + participleCertainty that something did not happenHe couldn’t have gone to the concert because he was doing the test.
Would have + participleDesire to do something in the past which in fact could not be done. I would have gone to the party, but I was too busy. Should/ought to + participleCriticism or regret after an eventYou should/ought to have warned me earlierShouldn’t have + participleCriticism or regret after an event, showing that it shouldn’t have happenedHe shouldn’t have forgotten about her birthday
Needn’t have + participleAn unnecessary past actionYou needn’t have brought anything to my party.
Should /Had better Should/had betterHad better is used in a more colloquial way of expressing what someone has to do, to give advise or opinions. You’d better go to the doctor. It also it is used to express a warningYou’d better tidy your room now