Let us begin by examining how we express the same idea with pronouns other than "I."
- If the pronoun were "you," we would say, "You're supposed to do that, aren't you?" corresponding to "you are" and "you aren't."
- If "she" were the pronoun, we would say, "She is supposed to do that, isn't she?" corresponding to "she is" and "she isn't."
In other words, in tag questions like these, we use the negative contraction of the verb "to be":
|Uncontracted Positive||Verb Contraction|
|I am||I . . . ?|
|you are||you aren't|
|s/he is||s/he isn't|
|we are||we aren't|
|you are||you aren't|
|they are||they aren't|
Notice that we have an imperfect paradigm: the negative contraction for "am not" seems to be missing. All the pronoun forms correlate to a positive form and a negative (contracted) form of the verb "to be"-all except one; there is no contractable form for "am not". Why not? The contraction of are+not is "aren't". The contraction of is+not is "isn't". Why isn't the contraction of am+not "amn't?"
In fact, in a wide swath of English dialects, it is. This word is common in Scotland and Ireland: "I amn't sure what he said" and "I am going, amn't I" are common in those variants of English. English doesn't like two nasal consonants like "m" and "n" together, however, and in most dialects they merged into "an't", the spelling of which eventually evolved into "ain't". "Ain't" then acquired the reputation of a "four-letter" word it has had to endure over the course of the last century. (Hmm. Actually, it has something in common with four-letter words, doesn't it?)
These rather pertinent facts of the English language were overlooked by the prescriptive grammarians who have all these years attempted to totally obliterate "amn't" and "ain't" from the English vocabulary.
"Never, never, never under any circumstances say 'ain't'" has been drummed in our heads since kindergarten. Sorry, teachers and diligent mommies, but this very simple rule is linguistically wrong. The rule that should have been drilled into the heads of English-speakers all these years is this:
|Only use "ain't" with "I"|
It is true that we should not say, "you ain't", "they ain't", or "she ain't" but we should say, without a scintilla of shame, "ain't I?" rather than "aren't I". "Aren't I" is just as ungrammatical as "I aren't" where "ungrammatical" means that it violates the rules, the speaking conventions, of the English language.
Now, don't go in and try to impress your teacher or your boss with this nugget of knowledge first thing tomorrow morning. We all have to coordinate the restitution of "ain't" to its rightful position in the language. That isn't likely to happen by daybreak but it is time to start thinking about it. Too bad the president didn't pardon "ain't"; it is a much more deserving four-letter word than "Rich". At least it is nice to know why this word refuses to go away despite the all-out assault it has suffered over the past century.