I ain't going to no school today.
She supposed that she ain't his lover anymore.
Though Jack insisted, Jill ain't moved the garbage.
As evident from these three non-standard sentences, including the double-negative one, 'ain't' is a tried-and-tested short form for pretty much all of the auxiliary verbs, except the modal auxiliaries.
'Ain't' was originally conceived in the 1700s, as a better usage of 'an't', which was the shorter use for 'are not' and 'am not'. These usages underwent their share of criticism over the centuries, for being non-standard and inelegant. But it found profuse usage even among the people of the upper class. The user needn't worry much about grammar rules since 'ain't' can be used for both plural and singular forms.
There is nothing wrong in using 'ain't' in fiction dialogs and informal conversation, but stick to the standard auxiliaries in all formal occasions. Here are a few sentences from great writers:
'I suppose yer the new boy, ain't yer?' said the voice through the keyhole. Charles Dickens, Oliver Twist
It's easy to criticize," I says, "but you ain't got the same problems as what I got."' George Orwell, 1984
'I made this journey after the '93 bombing, and I can tell you, lady, you ain't seen nothin' yet.' Jeffrey Archer, False Impression
"I hope it ain't your mother's middle name. Lot of people don't remember." John Steinbeck, East of Eden
Messenger, do please lend a careful ear. Your monophilia, your one one one, ain't for Jahilia. Return to sender. Salman Rushdie, Satanic Verses.
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