hmmm ok i try help u,,, Can and could have (or more precisely, should have) distinct differences, but these differences are often obscured by popular usage. Generally speaking, can means "to be able to" while could means "is possible, has the potential to". Therefore, in your example, "The phone is ringing—it can be your mother," while syntactically correct, is illogical because it implies that the phone (or the ringing of the phone) is itself capable of being your mother. "It could be your mother" is correct because the statement means, "It is possible that the caller is your mother." May, in addition to its other uses (to have permission to, etc.) basically means "to be possible". It can, therefore, be used in many contexts the same as could, e.g. "The phone is ringing again—that may be your mother." There is a subtle difference between may and could in that could also implies potential for ability not present, as in "The athlete could run faster if he exercises more." While one could also say, "The athlete may run faster if he exercises more," also meaning that there is a possibility of the athlete’s running faster if he exercises more, the former sentence carries the additional connotation that the athlete currently lacks the ability to run faster, but does have the potential for that ability. Here may only implies possibility. i hope this help u,,,=)

Here's a brief overview of the meanings of can, could and may with examples included Can - ability (ex Can you play the piano?) - theoretical possibility ( Anyone can make a mistake) - permission ( Can I leave early please?) - requests ( Can you give me a hand?) - offers ( Can I help you out?) Could - past ability ( Before the accident he could run very fast. ) - possibility (There could be trouble if the government tries to force this measure through) - requests (Could you lend me a hand?); unlike can, it is more formal. - asking for permission ( Could I leave a bit early today?); also used in a more formal context. - permission in the past ( When I was young, I could stay up late at weekends if I promised to be good.) - deduction ( That could be John's car I can hear - he said he was coming) - idiomatic ( You could at least tell me what they said [= I think you should...]) May - possibility ( We may go to France next year. ) - expressing hopes ( May you both be very happy) - permission ( You may go when you've finished.) - offers ( May I be of any assistance.); in this case may is more formal than can. - concession ( They may live next door but we hardly ever see them. ) As you can see, all three modal verbs can express possibility however can is the most informal. As regards could and may, the second one is used for actions which are more likely to happen, while could is usually used to emphasize a higher degree of unlikeliness. I hope this helps you.

Can- present tense (def. of can: is able to) (ex. can i do it?, i can do it., can you do it?, he can hit the ball.) Could- past tense (used as a possibility) (ex. i could do it., she could play it perfectly.) May- future tense (used as a possibility) (you usually use may when you're asking permission) (ex. may i...?, yes, you may.) website information that may help: http://www.onestopenglish.com/section.asp?docid=146362 http://www.yourdictionary.com/community/forums/viewthread/1797/ http://uk.answers.yahoo.com/question/index?qid=20080916125339AAO4JwN http://www.englishforums.com/English/DifferenceBetweenCould/zkmpz/post.htm

Can = 可以， Could is a past tense of can. For eg, Can you pass the test? You could have past the test if you studied hard. May is used in areas of possibility. Eg, May I help you? Dictionary may help you with your English.

can: it is used to say you can do something could: past tense of can; or that you can probably do something may: it is used to say that you might do something. It's similar to "maybe". I can write. ( present) I could write. ( past) I may write you a letter. ( it means that maybe you will write the letter)