Could, can, and would can be confusing in English. Editor Kory Stamper gives an explanation of how they are used.
Could, would, and can are all modal verbs (for more on some modal verbs, click here), and they can be difficult to master. Let's look at each one separately, and then compare some of their more confusing uses.
Can has many uses as a modal verb, but there are three uses that can be confusing to an English learner. These are: when can is used to describe ability ("I can change the oil in my car without help."), ask for permission ("Can I go to the movies tonight?"), or say whether something is possible or not ("Do you think he can win the race?"). Can is also used to suggest something might happen in the future ("If you finish your homework, we can go to the movies.").
Can, like could and would, is used to ask a polite question, but can is only used to ask permission to do or say something ("Can I borrow your car?" "Can I get you something to drink?").
Could is the past tense of can, but it also has uses apart from that--and that is where the confusion lies. When could is used as the past tense of can, it refers to an ability that a person generally had in the past or to something that was generally possible in the past ("When I was younger, I could run for miles," or "It used to be you could buy lunch for a dollar.").
Like can, could can be used of possibility as well, but the connotation is slightly different. In the sentence "We can have as many as ten people for dinner tonight," can is used to show ability ("We are able to have as many as ten people for dinner tonight;" “We have enough food/space for ten people.”).
By contrast, when could is used in this way, it refers to something that you believe is likely to be true or to happen. In the sentence "We could have as many as ten people come to dinner tonight," could is used to say that it is possible that ten people will come to the speaker’s home for dinner ("I think that it is possible we will have as many as ten people for dinner tonight.").
Could is also used to refer to something that you wish to have or do but that is not possible ("If only we could be free of this tyrant!"). It’s also used to describe something that was possible but did not happen, used with the verb have ("We could have won if we had practiced harder."). Could can also express annoyance or another strong emotion ("He could have asked me if I needed help!" or "I could have died I was so embarrassed!"), but can isn’t used this way.
When making general polite suggestions or asking a question, both could and can are possible ("Excuse me, can/could you tell me what time it is?").
Would is a little easier to understand because it is not related to could or can, and its uses are more specific. Would is used to talk about a possible situation that has not happened or that you are imagining ("I would quit my job if I won the lottery."), and it is also used with have to describe a situation that could have happened but did not ("She would have eaten less if she had known there was going to be dessert.").
Like could, would is used to describe something that you think is likely to be true or likely to happen ("I think the meal would [=could] feed ten people." "He said he would help me with this set of exercises when he gets home.").
Would can also be used to ask polite questions ("Would you mind if I had another cup of tea?"), or to wish for something ("I wish she would write a book."). It has several distinct uses that can and could do not, however: it is used to give opinions ("I wouldn't do that if I were you," or "I would say that she is in her 30s. [=it is my opinion that she is between 30 and 39 years old]"); it is used to express a willingness to do something ("I would be happy to take her to the airport"); and it is used to express frustration with something that you believe a person always does ("Well, he would say that, wouldn't he? He always says that!").