word choice - What's the difference between "should" and "could" (in the present tense)?

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.

Answer

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!)