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Could you please stop smoking here.

Is the sentence above an interrogative clause, but not a question?

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It is question, and needs a question mark at the end.

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can and could questions are a way of encoding commands to make them more polite. You don't really want to know anything about the ability of a person to perform the requested action. You already know that he/she can/could perform that action. The other person also knows this.

So "Can you pass the salt?" just means "Pass the salt, please", but it's more polite.

CJ