LZ你的理解是完全正确的!
ain't:[ eint ]
(=are not,am not)
“Ain’t” has a long and vital history as a substitute for “isn’t,” “aren’t” and so on.It was originally formed from a contraction of “am not” and is still commonly used in that sense.Even though it has been universally condemned as the classic “mistake” in English,everyone uses it occasionally as part of a joking phrase or to convey a down-to-earth quality.But if you always use it instead of the more “proper” contractions you’re sure to be branded as uneducated.
例：I ain't ready.（我还没准备好.） 例：I am very nice to you,aren't （我对你很不错,是不是?） 美国人用“ain't”很随便,应该是“are not,is not,have not,has not”的地方,他们也会“ain't”一下就带过去,宜注意.