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I am not referring to could as a past tense of can

What is the difference between the following sentences?

You should do it.
You could do it.

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should vs could. It is still the past tense of can. –  Matt Эллен Dec 4 '12 at 11:02
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closed as general reference by MετάEd, Daniel, tchrist, Matt Эллен, JSBձոգչ Dec 4 '12 at 19:47

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4 Answers

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The short answer is should implies either moral stricture or recommendation (it is desirable that you do it, either morally, or for your personal benefit). You should not swear. You should eat regularly.

The could form normally means you're being informed (or reminded) it is possible for you to do it, without necessarily implying anything about whether the speaker cares what you do, or whether it's beneficial to anyone. You could have banana flavour. You could be called up for jury service.

However, there is a standard 'idiomatic' usage wherein You could do it means exactly the same as You should do it.

Some would say that in that idiomatic usage, the word could actually amplifies the command (or criticism, for failing to have already done something). Using could in this way often implies a degree of exasperation on the part of the speaker; often meaning that the person being addressed is simply lazy (because he hasn't done whatever is being spoken of).

LATER: Here's one context where they don't mean the same. Suppose your car won't start, and a friend is standing next to you while you have the bonnet up trying to figure out what to do.

You should clean the spark plug leads implies that your friend thinks it's quite likely doing this will fix your problem, whereas

You could top up the oil implies your friend doesn't really know if that will do any good, but he thinks it might (and implies that he's probably not a motor mechanic!)

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That's true, "You could do it" can mean the same -- I overlooked that one. –  thursdaysgeek Jun 20 '11 at 17:40
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Should is used to indicate obligation, duty, or correctness, typically when criticizing someone's action; it also used to give advices.

I don't want to apologize to him.
You should do it.

Could is used to express possibilities.

What can I do? Do I persuade her supporters to support me?
You could do it.

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+1. Note that the possibility expressed by "could" is somehow "less likely" than the possibility expressed by "can": I can count to 10 (and I probably will); I could count to 1000 (but I probably won't). –  psmears Jun 20 '11 at 18:51
@psmears: Hmm. I can count to a million, even though I've never actually done it, and I'd bet any money I never will. –  FumbleFingers Jun 21 '11 at 0:45
@FumbleFingers: To me, the phrase "I can count to a million" invites the response "Go on then, prove it" ;-) –  psmears Jun 21 '11 at 9:07
@psmears: We can do it by statistical sampling. You give me a random number 0-999999, and I'll tell you the next number. As often as you like until you believe I can :) –  FumbleFingers Jun 22 '11 at 1:00
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Could is the simple past tense of can. Can implies possibility.

Should is the simple past tense of shall. Shall implies obligation or compulsion, arising either from moral duty or simple expediency. This verb contrasts with will (and would, its simple past), which expresses desire or purpose, but without certain intent or outcome.

Each of these words is also used to add a sense of conditionality or something like the subjunctive found in the romance languages but now largely defunct in modern American English (only affecting conjugation in the 3rd person singular).

So could, ultimately, expresses a possibility, perhaps merely conditional. Should expresses an obligation but with a somewhat softened effect compared with shall, which is also largely archaic in modern American English.

Presuming each describes a present scenario:

You should do it, means "You have some degree of compulsion to do it, arising from a duty, obligation, or best interest." People often speak this way meaning only, "I think it would be best for you," and people are sometimes irked to be spoken to so.

You could do it, means "You have the ability to do it," and might also carry the sense that there are contrary reasons for not doing it.

As ways of expressing the opinion that one's listener or reader should do it, the first is stronger than the second, and stops just short of being a command. The second, by contrast, states only the possibility, and pulls back even to the point of being merely an observation.

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"You should do it" means that it would be to your advantage if you do it.

"You could do it" means that you are able to do it, or that you may or may not do it but that the choice is yours and there is no advantage either way.

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you "would be" in "would be to your advantage if you do it" because you no it is NOT going to be to your advantage? –  Anderson Silva Jun 20 '11 at 17:34
Do you mean "because you know..."? "You should do it" is reommending the person to do the action. "You could do it" as both I and @FumbleFingers note, can have several meanings. –  thursdaysgeek Jun 20 '11 at 17:42
Actually, I was a bit hasty saying I agree your 'literal' definitions. "You should do it" doesn't normally mean it would be to your advantage - it normally implies 'external' moral pressure to do something. Context is everything though, and here we have very little to go on. Given what we have, I think the alternative to "they're the same" is that the first implies the speaker is pretty sure "doing it" is the best option, whereas in the second he's a bit uncertain. –  FumbleFingers Jun 20 '11 at 17:52
I can agree with that: the speaker thinks that it is something that should be done; whether it is truly to the advantage of the listener, or only the speakers opinion is not known. –  thursdaysgeek Jun 20 '11 at 17:57
@thursdaysgeek: I think FumbleFingers' point is that though you're right that the should might be to the advantage of the listener (You should check your tyre pressure!), often it is to the advantage of the speaker, or society at large - possibly to the detriment of the listener (You should give more money to charity!). –  psmears Jun 20 '11 at 18:55
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