A question tag is a short question form repeating the main clause verb and thereby creating either a question to which the speaker thinks he knows the answer, but is not sure; or a request for affirmation, depending on the sentence inflection:
'It's hot today, isn't it?' This tag, with a falling tone at the end of the sentence, is asking the listener to affirm that the weather is very warm.
'You're hungry, aren't you?' This tag, with a rising tone at the end of the sentence, is a question; the speaker thinks the listener is hungry, but is not sure.
The tag verb is the same as the tense verb of the main clause:
'They have gone, haven't they?'
'We should study, shouldn't we?'
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