Subject: Can vs could
I received this question from a former student in a recent email:
How could I explain the difference between the verb "can" and "could"?
Does "could" involve with one's willingness to do something and "can"
is talking about one's ability?
Here's my answer:
I can understand your confusion. There is more than one meaning for "can" and "could."
"Can" generally refers to ability ("to be able to do something")
I can walk a mile in twelve minutes. = ability
I can speak Cantonese. + ability
"Could" = past ability
When I was 16, I could walk a mile in 8 minutes.
At one time I could speak Cantonese, but now I have forgotten how.
Sometimes "can" is used to ask permission, though some strict grammarians say this is wrong.
Can I go to the bathroom? = May I go to the bathroom?
A strict grammarian would say here that "can" is wrong and makes the request about ability instead of permission. HOwever, "can" is NOW widely used in the US to ask permission. I would argue that language is a living thing, and although it may be wrong to some people, most are using it that way.
"Can" and "could" are sometimes used to make a polite request:
Can you help me?
Could you help me?
Would you help me?
The last two are considered more polite than the first by some.