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Question tags are the tiny questions English speakers add to the end of sentences to invite agreement or keep a conversation going. French has n'est-ce pas ? and non ? — but English has dozens of variations, and using them makes you sound dramatically more natural.
Positive sentence → negative tag: "It's warm, isn't it?"
Negative sentence → positive tag: "You haven't been here before, have you?"
The tag always uses the same auxiliary verb as the main sentence (is/isn't, do/don't, have/haven't, can/can't).
French speakers often use "no?" or "isn't it?" for everything. But the tag must match the sentence: "You came yesterday, didn't you?" — not "isn't it?". The auxiliary changes with each sentence.