For some people, innit is just another tag question, a contraction of isn’t it. But kids in urban Britain are using innit to cover a wider and wider range of situations. Here are some examples of non-standard use, gleaned from recent messageboard postings:
In the first example, innit can be seen to replace a tag question like don't we?. In the second example, innit might be interpreted as can't I?. In the third example, innit could mean aren't they?; in the fourth example won't I?; and in the fifth example, can't he?.
In Standard English, tag questions refer back to the Subject of the full sentence. Many languages have a formal, standard tag question called an invariant tag question. The invariant tag question doesn't change in relation to the Subject of the full sentence. Invariant tags are common in other languages: Spanish has ¿verdad? and ¿no?, German has nicht wahr? and the non-standard oder? and French has n’est-ce pas? In non-standard English, innit is an invariant tag question, similar to the invariant tag questions in many other languages.
Is innit ungrammatical? Its use varies between different groups of speakers, and many individuals will have their own consistent (but usually unconscious) grammatical rules on when innit can and can’t be used. For some, innit is invariant – while others will use both invariant innit and a range of other tag questions, depending on the situation.
The best linguistic answer is to point out that innit is non-standard, so it shouldn't be used in formal essays and it won't be useful in a job interview. Nonetheless, innit is fascinating for linguists studying the way that English changes over time.
Invariant tags can also be heard in varieties of English spoken in Papua New Guinea, Singapore and South Africa. In fact, it’s likely that the current use of innit in the British Isles has spread from certain immigrant groups of speakers in London to some of the wider population.