To a question on Quora about the “difference between ‘I am’ and ‘am I,’ I submitted this answer:
Inflections in a language are changes within words that indicate attributes such as tense, case, number, gender, and so forth. For example, the English-language suffix -ed to show past tense is an inflection.
English uses few inflections compared with, say, German, which is said to be “highly inflected.” Instead, English relies upon word order. The statement “I do play the trombone” has a meaning quite different from the question “Do I play the trombone?”
Thus, “I’m” (or “I am”) is understood in English to begin a statement, whereas “Am I” usually introduces a question. Interestingly, you will rarely hear English-speakers say “Am I not?” Someone arriving tardily to a meeting will rush into the room, panting, “I’m late, aren’t I?” It’s ungrammatical, strictly speaking, but the logical contraction “amn’t” does not exist in English. “Aren’t I” is acceptable in virtually every context.
What I did not say, because it wasn’t germane to the question, is that the much-maligned word ain’t could slip neatly into the first-person-singular negative interrogative form of the verb to be. I would go so far as to say that “ain’t I” is better, grammatically speaking, than “aren’t I.”
When I was learning the language, ain’t was the grammatical scarlet A. It scorched the air like a cussword in a deacons’ meeting. A person who said “ain’t” was not only linguistically inept but also considered intellectually backward and socially inferior, one of the great unwashed, fortunate to have shoes and clean underwear, probably living in a rusted-out trailer, three kids to a room. Ain’t is probably the most stigmatized word in the English language.
No one is sure why this is so, as, indeed, ain’t was standard for centuries among cultured speakers in literature, particularly in Britain. “For most of its history, ain’t was acceptable across many social and regional contexts. Throughout the 17th, 18th, and 19th centuries, ain’t and its predecessors were part of normal usage for both educated and uneducated English speakers, and was found in the correspondence and fiction of, among others, Jonathan Swift, Lord Byron, Henry Fielding, and George Eliot.” (Wikipedia)
Logically, we might as well say, “I amn’t.” It would be consistent with the second and third persons, as in, “You aren’t,” and “he isn’t.” But the issue doesn’t arise in the declarative form because we contract I with am and say, “I’m not.” Only in the interrogative do we come up against the lack of a contraction that makes grammatical sense, and so, rather than say, “I’m late, am I not?” which is just too, too prissy for us plainspoken Americans, we blurt, “I’m late, aren’t I?”
And we’ll keep on committing this same solecism, as long as the grass is green and the skies are blue, because, thankfully, language is not math and there are quirky inconsistencies at every turn. Are there not? And would we truly have it any other way?