There are instances in which could can refer to the
future and has the same meaning as can:
1. As a polite request:
|
(a)
|
Can/Could you help me, please?
|
2. As a suggestion:
|
(b)
|
There are other ways to get there. You don't have to cross the
bridge. You can/could take the ferry, or you
can/ could drive around the whole bay.
|
In sentences (a) and (b) above, either can or could
may be used.
3. As part of a conditional sentence, the meaning is different with
could:
|
(c)
|
Well, I could go with you.
|
A complete sentence might be:
|
(d)
|
Well, I could go with you if you really wanted
me to.
|
In sentences (c) and (d), changing could to can alters
the meaning of the sentence; you would have to alter the if-clause,
too:
|
(e)
|
Well, I can go with you if you really want
me to.
|
The sentences in (3)—sentences (c), (d), and (e)—do refer
to the future, but can and could are not interchangeable
here.
RSK
As Rachel notes, can and could have the same meaning
in polite questions, with could being considered the more polite
and more formal:
| (a) |
Can you please open the door for me? |
|
(b)
|
Could you please open the door for me?
|
They also have a very close meaning in the negative:
| (c) |
That can't be true! |
|
(d)
|
That couldn't be true!
|
| (e) |
She can't have meant
to insult you. |
|
(f)
|
She couldn't have meant to insult you.
|
In reported speech, there's no real difference in meaning; the difference
lies in immediate vs. later reporting of what was said and in the level
of formality the speaker chooses, with the past form being the more
formal.
| (g) |
He said he can help us tomorrow. (immediate
or informal reporting) |
|
(h)
|
He said he could help us tomorrow. (later or
more formal reporting)
|
To express possibility, could usually carries with it the idea
of an implied condition. Using Rachel's example:
| (i) |
You could take the ferry (if you
wanted to, if that seemed like a good idea to you). |
|
(j)
|
You can take the ferry. (a factual option)
|
Explaining to a non-native speaker the difference between sentences
(i) and (j) would be no easy task! I've found that we can't
and indeed shouldn't try to explain to students too finely nuanced
differences
in modal usage—but perhaps others have a different teaching
experience?
While it's true that all modals are complicated, with varied usages
and lots of nuances, it seems to me that could might possibly
be the most complicated of all in terms of its many meanings and uses.
For me, the best sources on the meanings and uses of modals are dictionaries.
The Collins COBUILD English Language Dictionary is a good source
for teachers and researchers. For students, the Collins COBUILD English
Dictionary for Advanced Learners is quite complete, and the Longman
Advanced American Dictionary and Oxford Advanced Learner's Dictionary
are quite accessible and useful for students' purposes.
Betty Azar