Can vs Could

I went to teach English in Japan without any formal teaching qualification. I have always been interested in grammar. I was often confused as to how the textbooks were teaching many aspects of English. I was not happy about the way can / could was taught but I could not pinpoint why.

‘Can’ vs ‘Could’. These words are used when asking for permission, making requests and about ability or possibility.

The differences are usually described like this – ‘could’ is the past tense of ‘can’ when talking about ability or possibility.

When talking about asking permission or requests ‘could’ is thought of as more polite than ‘can’.

I managed to read one guy who wrote that actually the difference between ‘can’ and ‘could’ is distance. In other words we use ‘can’ when the concept or person is close to us, and ‘could’ when it is far. This ‘rule’ seems to fit all the differences between ‘Can’ vs ‘Could’. So why is it not taught? For example - ‘could’ is used in the past tense of ‘can’. The past is far and now is close. You ask permission from a professor by saying ‘Could I borrow a textbook?’ but from your friend ‘Can I borrow a book?’

Your professor is far and your friend is close.

Can anyone tell me who this I was I read that explained the ‘Can’ vs ‘Could’ difference?

My answer is not specific to can/could, but I first heard tense described as a matter of “distancing” in this paper (pdf file). I found the link at the UsingEnglish.com forums.

There is a list of “further reading” at the end of the article. Maybe you came across the idea in one of those sources.

Here’s what I’ve come up with:

’could’ is hypothetical:
I could help you with your homework if I knew more about maths.
I could help you with your homework if we can find a mutually agreeable time.

’can’ is about ability:
I can help you with your homework, because I know a lot about maths.
I can help you with your homework tonight after dinner.

’will’ is about agreement/commitment:
I will help you with your homework, because you asked me so politely.
I will help you with your homework tonight after dinner.

’could have’ is the past tense of both ‘can’ and ‘could’:
Why didn’t you tell me you had maths homework last night? I could have helped you because I know a lot about maths.
I could have helped you with your homework last night if I hadn’t been so busy.

’would have’ is the hypothetical past tense of 'will’
Why didn’t you tell me you had maths homework last night? I would have helped you.

ETA:

’may’ is the “requesting permission” version of 'can’
May I have another sausage?
Yes, you may.

’might have’ is another hypothetical past tense
I might have helped you if you’d asked me politely.

I might enjoy a can of beer after a long day. I mean, I could. Cans are also easier to return than bottles. (If only I could get around to returning all these cans, this place is a mess.)

When you’re referring to ability or possibility, the can/could distinction gets even odder, I think. It devolves into a matter of willingness, intensity, and certainty when discussing these factors.

“We need some nylon twine and some ex-lax.”

[Sure,] I can go to the store to pick that up. – Acknowledgement that the items are necessary, and willingness to perform the action.

I could go to the store to pick that up. – Either dubiousness that the items are necessary, or slight hesitation to perform the action (though it also is more willing than neutral).

I COULD go to the store to pick that up. – Intense dubiousness that the items are necessary, or general unwillingness to perform the action (though still willing if deemed necessary by peer/group/superior). Oftentimes this construct is followed by “… but [argument/excuse]”

I CAN go to the store to pick that up. – For some reason I don’t feel this has the element of dubiousness that the COULD construction has, but it contains greater unwillingness to perform the action, unless absolutely necessary.

Note that this is pretty colloquial/informal speech and probably not suited to just learning the language, since the “technical” definitions of these sentences has to do with tense.

FWIW, I started a thread a while back (wow, was it really that long ago?) asking a similar question about “may” vs. “might”.

But it’s also less definite. “Would have” is definite.

Also, “may” can also be used as a hypothetical future tense. “I may help you if you ask me politely.”

I always think of can/could in this way:

Can = Permission to
Could = Ability to

My study of languages has forced me to think in terms of rules, even though contemporary English, for the most part, defies them.

I think of both of them as tenses of “having the ability to”

I think the more important distinction to draw is between ‘can’ and ‘may’. ‘May’ implies permission or lack of it, whether granted by a person ir granted by the fact that certain conditions are met.

‘Might’ is an indeterminate form of ‘can’ and IMO is not related to permission.

There’s a lot of nuance - for example:
“Can you hold this guy down while I chop his leg off?” Are you available, are you strong enough?
“Could you hold this down while I chop his leg off?” Are you gonna barf on me or what? Do you have what it takes?

I use ‘can’ and ‘may’ interchangeably, with no distinction other than ‘may’ is politer than ‘can’. ‘Might’ is politer still, and has the same usage as ‘can’, that of requesting permission.

An analysis of “can” versus “could” in terms of present versus past tense is misguided, because that’s not the difference here. “Could” can be past tense (e.g., “I could touch my toes when I was younger, but I can’t now”), but in the contexts given in this thread, “could” is the subjunctive mood, not the past tense. So it’s used to indicate a hypothetical situation and in a request (“Can you do this” versus “Could you do this”) it is therefore the politer form to use.

Of course, a lot of languages (including Japanese) don’t have a subjunctive mood, so they accomplish the same thing by different means, e.g., by using a polite form of the verb. However, the subjunctive mood is not necessarily used for politeness: it’s used mostly to indicate nonreality or hypotheticalness.

Correct:

That Kim Kardashian sure has nice cans.

Incorrect:

That Kim Kardashian sure has nice coulds.

Okay:

I could get canned for saying that aloud at work.

This is really the significant difference. “Can” implies the immediate ability to perform an action. “Could” implies the potential ability to perform an action if some condition or other is met.

To pontificate a bit, I would imagine this is where the ‘softening’ use of could over can comes into play. “Can you drive me to the store?” is somewhat direct and presumptive (if you can, then obviously you will). “Could you drive me to the store?” asks if one has the ability with the additional implication that that ability is contingent on their desire to. It leaves room for that condition to not be met.

What kind of movies do they show at the Could Film Festival?

That’s a hypothetical question. :smiley:

Well, that would make you what I like to call… wrong! :smiley:

I can do something means I am ready and able to do it.
I could do something means it would be possible, but there is a condition to doing it.

I can show you how to use Photoshop! (We have the computer and software and the time!)
I could show you how to use Photoshop! (Unfortunately, we do not have a computer or the software or the time.)

If you go to Paris, you can see the Eiffel Tower! (Hard to miss when you are in the city!)
If you went to France, you could see the Eiffel Tower! (Of course, you would have to go to Paris to see it.)