Could is a modal verb. It is used with the base form of a verb. Could is sometimes considered to be the past form of can, but in this dictionary the two words are dealt with separately.
1 modal You use could to indicate that someone had the ability to do something. You use could not or couldn't to say that someone was unable to do something.
For my return journey, I felt I could afford the extra and travel first class..., I could see that something was terribly wrong..., When I left school at 16, I couldn't read or write..., There was no way she could have coped with a baby around.
2 modal You use could to indicate that something sometimes happened.
Though he had a temper and could be nasty, it never lasted..., He could be very pleasant when he wanted to.
3 modal You use could have to indicate that something was a possibility in the past, although it did not actually happen.
He could have made a fortune as a lawyer..., He did not regret saying what he did but felt that he could have expressed it differently.
4 modal You use could to indicate that something is possibly true, or that it may possibly happen.
Doctors told him the disease could have been caused by years of working in smokey clubs..., An improvement in living standards could be years away...
5 modal You use could not or couldn't to indicate that it is not possible that something is true.
Anne couldn't be expected to understand the situation..., He couldn't have been more than fourteen years old.
6 modal You use could to talk about a possibility, ability, or opportunity that depends on other conditions.
Their hope was that a new and better East Germany could be born..., I knew that if I spoke to Myra, I could get her to call my father.
7 modal You use could when you are saying that one thing or situation resembles another.
The charming characters she draws look like they could have walked out of the 1920s.
8 modal You use could, or couldn't in questions, when you are making offers and suggestions.
I could call the local doctor..., You could look for a career abroad where environmental jobs are better paid and more secure..., It would be a good idea if you could do this exercise twice or three times on separate days.
9 modal You use could in questions when you are making a polite request or asking for permission to do something. Speakers sometimes use couldn't instead of `could' to show that they realize that their request may be refused., (politeness)
Could I stay tonight?..., Could I speak to you in private a moment, John?..., He asked if he could have a cup of coffee..., Couldn't I watch you do it?
10 modal People sometimes use structures with if I could or could I as polite ways of interrupting someone or of introducing what they are going to say next.
FORMAL, SPOKEN, politeness
Well, if I could just interject..., Could I ask you if there have been any further problems?..., First of all, could I begin with an apology for a mistake I made last week?
11 modal You use could to say emphatically that someone ought to do the thing mentioned, especially when you are annoyed because they have not done it. You use why couldn't in questions to express your surprise or annoyance that someone has not done something., (emphasis)
We've come to see you, so you could at least stand and greet us properly..., Idiot! You could have told me!..., He could have written..., Why couldn't she have said something?...
12 modal You use could when you are expressing strong feelings about something by saying that you feel as if you want to do the thing mentioned, although you do not do it., (emphasis)
`Welcome back' was all they said. I could have kissed them!..., She could have screamed with tension.
13 modal You use could after `if' when talking about something that you do not have the ability or opportunity to do, but which you are imagining in order to consider what the likely consequences might be.
If I could afford it I'd have four television sets..., If only I could get some sleep, I would be able to cope.
14 modal You use could not or couldn't with comparatives to emphasize that someone or something has as much as is possible of a particular quality. For example, if you say `I couldn't be happier', you mean that you are extremely happy., (emphasis)
The rest of the players are a great bunch of lads and I couldn't be happier..., The news couldn't have come at a better time.
15 modal In speech, you use how could in questions to emphasize that you feel strongly about something bad that has happened., (emphasis)
How could you allow him to do something like that?..., How could she do this to me?...
Could've is the usual spoken form of `could have', when `have' is an auxiliary verb.