We use could to talk about past time:
She could speak several languages.
They couldn’t dance very well.
We tend to use could as the past tense of can to talk about ability in the past:
I could hear Beth sniggering and cringed in embarrassment.
Mozart could play the piano blindfolded.
By the 1970s, jumbo jets could fly almost anywhere non-stop.
We use could have to refer to something that was an option or generally possible in the past but didn’t happen:
She never stopped daydreaming about the life she could have lived in Greece.
If you’re referring to a general past situation when something was allowed, use could:
The Americans were under instructions that no-one could smoke indoors.
To refer to a past unrealistic situation or strong inclination, use could have:
She was so thirsty, she could have drunk a gallon of water.
He irritated me so much that I could have screamed.
Could have is also used (in a similar way to might) to show annoyance when you think someone should have done something, but they didn’t:
You could have told me that she wouldn’t be at work today!
Can and could versus may or might
This section provides more information on some points outlined in the may and might blog, concentrating on the way these verbs are used to make offers and requests and to ask for and give permission.
1.) Requests and offers
When making a request for something, the most usual way to do this in everyday English is to use can or could:
Can I have two coffees please?
Could I have two coffees please? [more polite than can]
Although can and could are perfectly acceptable, some people prefer to use may in such cases, as it’s regarded as more polite and more formal:
May I have two coffees please?
Nowadays, using might to make requests is generally reserved for very formal situations and to make the request sound more like a polite suggestion than a firm instruction:
Might I ask the Court to glance briefly at the judgment of Sir Harry Gibbs?
When making an offer, can is the most frequent way of doing this in everyday English; could is used when we want the offer to sound more tentative; may is more formal and more polite:
Can I get you another drink?
Could I help you in any way?
May I get you another drink?
2.) Asking for and giving/refusing permission
The most typical way of asking for permission in today’s English is to use can, or if you want to sound more polite, could:
Can I borrow your pen?
Could I borrow your pen?
Although this is part of standard English, many people believe that can and could are incorrect within the context of permission and should be reserved for talking only about ability and possibility, and thus it is advisable to use may in more formal writing and speaking (might is regarded as very formal):
May I borrow your pen? [polite, formal]
Might I borrow your pen? [rare, polite, very formal]
1used as the past tense of 'can'
Renee could already read when she was four.
In the distance I could see a cloud of smoke
YOU think that reported speech doesn't mean past tense. That's you being indiosyncratic. Not what others think.
Has Jack gone to London, Jack? Does "he could go to London" signal that Jack has gone to London?
No, but it does signify that the statement was made in the past.
It was in the past, that's why could was used.
jtt: Has Jack gone to London, Jack? Does "he could go to London" signal that Jack has gone to London?
Izzy: No, but it does signify that the statement was made in the past. The immediate past, but still the past.
You overcomplicate everything,
Let's recast JTT's imaginary dialogue a bit:
JTT (yesterday): So, can you go to London?
Sally (yesterday): Yes, I can go.
JTT (today): So, yesterday I asked you if you could go to London, and you said you could, but you didn't use those exact words, did you?
Sally (today): No, I said 'I can go', not 'you could', but I definitely could go, I'd asked me mum and she said yes.
<JTT gnashes teeth>
It's not reported speech, it's actual (if imaginary) speech,
and what are you doing trying to lure underage girls to London, JTT?
That's your point, JTT, not mine. I'll leave you to get on with it.
Izzy as a role model is a far more sensible aspiration than you are.