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Is it correct to use "could" here?

Did you know you could do that?

What grammar rule do I have to look at to reliably answer those questions myself?

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    I'm sure it is "grammatical", but I don't suppose most native speakers would be able to say exactly which contexts allow/require could as opposed to can. For many people and/or many contexts, they're interchangeable (meaning the hearer won't distinguish a difference in meaning or "correctness" any more than the speaker). – FumbleFingers Apr 9 '16 at 1:48
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When speaking of the present, or without reference to time (in general), the words are interchangeable to the native speaker. Using the word could as opposed to can is only more appropriate when the speaker wishes to emphasize that the capability being discussed isn't recent.

Let's discuss two employees: Goofus and Gallant. Goofus received a new clearance on his badge two months ago. Gallant just received that same clearance today and, unlike Goofus, took the time to read the instructions that came with it. It is raining very hard. As Goofus heads toward the street to walk into the building through the pouring rain, Gallant stops him, "Goofus, didn't you know you could use the covered entrance with the new clearance?"

Once again, the word could could be replaced with can without making the sentence ungrammatical, but by using the word could, Gallant is trying to drive home to Goofus that the capability is one he's had for quite some time.

So, to answer your questions in order:

Is it correct to use "could" here?

Answer: Absolutely

What grammar rule do I have to look at to reliably answer those questions myself?

Answer: Tense. When the capability isn't something recent, you could use the (simple) past tense, but otherwise, you can use whichever word you wish.

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