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So far, we’ve seen that could is often used as the past tense of can. Other important meanings and uses include the following.

•Use could (not can) to refer to conditional situations, in which something has to happen or be the case in order for someone to be able to do something or for something else to occur:

We could buy a new sofa if we stop eating takeaway meals every night.

--Source

I wonder why can cannot be used here. I would think if can were used, it would suggest capability or opportunity to do this in the situation, which would be quite reasonable.

Are there any other examples which can justify this usage?

Please help to clarify

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3 Answers 3

up vote 6 down vote accepted

In the Q-clause (consequence clause) of conditionals, can usually implies a fairly definite intention—under such-and-such actualized or about-to-be-actualized circumstances, we will at last be able to do something we’ve wanted to do.

Now that we’ve saved so much we can buy a new sofa.

Could is more tentative. It’s used to express a suggestion rather than an intention: under such-and-such circumstances we would be in position to consider doing something we’ve wanted to do.

If we stopped eating takeaway every night we would save enough that we could buy a new sofa.

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Also, could may often be emphasized: could (spoken as cou-u-uld). This may be done to "push" the listener's decision in the desired (by the speaker) direction. "We could buy a new sofa if...". –  Phil Perry May 14 at 19:27
    
Just one more question: why use stop rather than stopped in that example? While in your own, you used the conditional mood. –  Zhanlong Zheng May 15 at 0:45
    
@ZhanlongZheng Good catch; I missed that. Realis stop is a bit more 'open', a real possibility we could actually pull off as opposed to "Well, we could consider stopping ... " –  StoneyB May 15 at 1:05
    
Now that we've saved so much, we could buy a new sofa. Now that we've saved so much, we can buy a new sofa. They seem fine to me, but could seems more tentative, so agreed on that. –  jimsug May 15 at 13:58
    
Come to think of it, is "might" an alternative option for "could" here? Out of curiosity, why did up to five people upvote for this question? I thought it was just an ordinary one. @StoneyB –  Zhanlong Zheng May 28 at 14:30

The difference between "can" and "could" here is a difference in the verb's mood.

In the sentence:

We can buy a new sofa if we want to.

the verb phrase "can buy" is in the indicative mood. This means that you have the present ability to buy a new sofa.

In the sentence:

We could buy a new sofa if we had a hundred dollars.

the verb phrase "could buy" is in the conditional mood, indicating that it is dependent on the fulfillment of the condition given in the following clause.

This can be a subtle distinction. For instance, you would say:

We can buy a new sofa as soon as you win the lottery.

This seems, logically, like a conditional--but grammatically, at least, winning the lottery is not seen as a contingency here, because it is not phrased as one. In this case, the statement is sarcastic, and the sarcasm comes from the fact that "as soon as you win the lottery" is not phrased in such a way as to suggest a contingency.

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Consider these two sentences:

a) We could buy a new sofa, but I don't know if we have enough money. b) We can buy a new sofa, because we have enough money.

"Could" means that buying the sofa is uncertain to a small degree, but still possible to happen, depending if a set of conditions are met (if the characters manage to save enough monety)

"Can" means that "they" can perfectly buy the sofa, and that the possibility of doing so rests completely on the subjects' decision.

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