一般来说是is not
然而is not的缩写除了isn't以外,还有一个口语化但不太规范的缩写:ain't
在牛津高阶学习者词典第八版中,对于ain't的解释是:
1
am not/is not/are not
Things ain't what they used to be.
2 has not/have not
I ain't got no money.
You ain't seen nothing yet.
可见is not的缩写也可以是ain't。从这个角度来说,isn't=ain't