Can is a modal verb which denotes ability to do something and maybe replaced by to be able to. Ex. I can do it. I am able to do it. It also may mean a permition to do something. Ex. You can go home now. Or prohibition Ex. You cannot go outside unless I permit you. In negative sentences it may also mean a strong level of doubt Ex. She can't be at home now. In interrogative sentenses it may express surprise Ex. Can he still be working here? or a request to do something Ex. Can you do me a favour? Could expresses all the same as can but in the past tense. NB Could is used to express polite and/or formal request Ex. Could you do me a favour. Would is a form of will used in past tenses and has the same meanings. NB would is also maybe used to express polite and/or formal request Ex. Would you do me a favour?