Sometimes I encounter ain't, but I really don't know how to translate it properly.
What does ain't stand for? If I really wanted to use it, in which contexts would you say it's acceptable using it?
Ain’t is a negative present-tense form of the verbs be and have employed in all persons and numbers:
I ain't we ain't you ain't you ain't he/she/it ain't they ain't
It represents a coalescence of the ordinary spoken contractions of not and the three relevant forms of the two verbs:
am not ⊲ a’n’t ) are not ⊲ a’n’t ) is not ⊲ i’n’t ) ⊲ e’n’t/ha’n’t ⊲ orthographic ain’t/hain't have not ⊲ ha’n’t ) has not ⊲ ha’n’t )
It is used wherever be not is used: as a copula, in progressive constructions, and in passives; and where have not is used as an auxiliary, in perfect constructions.
Ain’t is not slang (which means, roughly, a fairly novel usage employed by an ‘in-group’ as a token of their ‘in-ness’) but a colloquialism which was at one time used virtually universally. You find it very often in 18th- and 19th-century plays and novels, in the mouths of persons of high social standing.
However, it aroused particular hostility among 19th century schoolmarms, who assaulted it ruthlessly and succeeded in painting it as the mark of illiterate speech. Accordingly, it should not be used in formal contexts except as an ironic nod to the vernacular. It is still very common in speech, but regarded as sub-standard rather than merely non-standard.
Ain't is used as a regular negated form of be or have, and supposedly sometimes do:
I ain't no tractor. = I am not a tractor. I ain't got no tractor. = I haven't got any tractor.
It's also used like there isn't, by common omission of there from there ain't.
Ain't no tractor here. = There isn't any tractor here.
And in case you hadn't guessed, dialects that use ain't stereotypically use negative concord as well.
—Jon Purdy (link)
The definition here says it all.
Contraction of am not.
Used also as a contraction for are not, is not, has not, and have not.
It may be that these extended uses helped fuel the negative reaction. Whatever the case, criticism of ain't by usage commentators and teachers has not subsided, and the use of ain't is often regarded as a sign of ignorance. · But despite all the attempts to ban it, ain't continues to enjoy extensive use in speech. Even educated and upper-class speakers see no substitute in folksy expressions such as Say it ain't so and You ain't seen nothin' yet.
So, basically, it's slang, but there are times when no other word fits. I wouldn't use it in a formal situation, except to quote a title. I'd use it sparingly except in very informal situations.