You use can when you are mentioning a quality or fact about something which people may make use of if they want to.
Could've is the usual spoken form of 'could have', when 'have' is an auxiliary verb .
could do with:
If you say that you could do with something, you mean that you need it or would benefit from it.
could care less:
feel (s) the least possible degree of interest, sympathy, etc.
sb could do worse:
If you tell someone that they could do worse than do a particular thing, you are advising them that it would be quite a good thing to do.
the least I could/can do:
You use expressions like ' that's the least that I can do ' to mean that you are very willing to do it, or to acknowledge someone's thanks .
as happy etc as can/could be:
If someone or something is, for example, as happy as can be or as quiet as could be, they are extremely happy or extremely quiet.
before you could say Jack Robinson:
said to mean that something happened very suddenly and quickly
couldn't care less could care less:
If you say that you couldn't care less about someone or something, you are emphasizing that you are not interested in them or worried about them. In American English, you can also say that you could care less, with the same meaning .
someone could wring someone's neck:
said to mean that a person is very angry with someone else
wish you could turn back the clock:
to wish to return to an earlier period, for example because you think it was a very good time or because you would like the chance to live your life differently
you could cut the atmosphere with a knife:
said to mean that the atmosphere in a place is extremely tense or unfriendly
you could have heard a pin drop you could hear a pin drop:
You can say you could have heard a pin drop when a place is extremely quiet, especially because everyone is waiting for someone to speak or when someone has made a shocking remark .
you could have knocked me down with a feather:
said to mean that you were extremely surprised or shocked by something
as far as the eye can see as far as the eye could see:
If there is something as far as the eye can see, there is a lot of it and you cannot see anything else beyond it.
more...than you could shake a stick at more things than you can shake a stick at:
If you say that there are more things than you could or can shake a stick at, you are emphasizing in a humorous way that there are a lot of them.