Tag questions (aren't we?, don't you?, won't she?) have the following properties (among others):
- They follow the main clause
- They have the form of a question
- They contain an auxiliary verb that matches the one in the main clause or is do (are, do and will in the examples above)
- The verb agrees in person and number with the main clause verb (e.g. 1st person plural, 2nd person singular or plural, and 3rd person singular in the examples above)
- They have the opposite polarity to the main clause (so they're often negative, following a positive main clause, but can be positive if the main clause is negated: He's not coming, is he?) (though see NB below)
- They have a pronoun that matches the main clause subject (we, you, she above)
Isn't it is a form of tag question, then: it has all the above properties in an example like It's a funny old world, isn't it?. But in some dialects, it has crept out of these constraints and is used in a broader range of contexts, and as well as losing a lot of its phonological properties (it's reduced to innit), some of the syntactic properties in 1-6 no longer apply to it.
1: It still follows the main clause. But that's about it.
2: It may not have the form of a question. Lots of times, it's written without a question mark (OK, that's no guarantee, but it's telling). It's attached to sentences that can't possibly be questioned, like assertions on the part of the speaker: You're fit, innit. That's the speaker's opinion. How can it be questioned?
3: The verb probably doesn't match the main clause one now - in the example I just gave, it does, but you can also say I've got no money, innit, where it doesn't.
4, 6: We no longer need the agreement in person and number. In We're late, innit, the verb in the main clause is first person plural. Innit, if it comes from isn't it, is 3rd person singular. Similarly, the pronoun is it rather than the matching one in the main clause, we.
5: Generally, the polarity is opposite to that in the main clause, but only by chance: innit is negative, and more sentences are positive than negative. But I heard an example on Gogglebox this week of a negative clause followed by supposedly negative innit:
That's not actually a dumb question, innit.Cool, innit?
NB: Tag questions can have the same polarity as the main clause if they're both positive, but it has a different meaning. Compare:
a. That's the bus we need to get, isn't it?
b. That's the bus we need to get, is it?
In a, the speaker thinks they have the correct information and wants their interlocutor to confirm it. In b, the speaker thinks that the interlocutor has the relevant information, and wants to confirm it. But we don't find negative assertions and negative tags:
*That's not the bus we need to get, isn't it?