They say this type of interrogative(see title) can function as polite requests, for example:
You couldn't carry this for me, could you?
However, as a non-English native, I cannot figure out why it can be a request when the main clause is negative.
To me, I would think the main clause should be positive together with a negative tag, whereby a polite request is formed. For example:
You could carry this for me, couldn't you?
Could you tell me why (negative clause + affirmative tage) can be a polite request?
Thank you very much indeed.
Politeness is more important than logic.
If I say "You could carry this for me" I'm assuming you can and will and want to.
If I say "You couldn't possibly... " then I'm assuming that I am too much trouble and then YOU say that of course you can. It allows the other person to take the lead on doing the favor instead of my assuming he will do it. (I don't assume anything - it's just how the dance goes.)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.