is not缩写形式是isnt。
与not相关的否定缩写：通常在原词后加nt，若原词以n结尾，直接加t。
系动词be、助动词、情态动词的否定形式一般可以缩写成为一种简略的形式。
相关介绍
如：
is not—isn't、are not—aren't、was not—wasn't、were not—weren't、has not—hasn't、have not—haven't、will not—won't、do not—don't、did not—didn't、used not to—usedn't / didn't use to。
1、含有 am is are的时候，使用缩写形式如：
I'm=I am；she's=she is；he's=he is；it's=it is；we're =we are；you're =you are；they're=they are；there're=there are；that's=that is；Sam's=Sam is。
2、含有have，has以及他们的过去式had的时候，常用缩写式如：
Weve got=we have got；You've got=you have got；they've got=they have got；she's got=she has got；he's got=he has got；it's got=it has got。
3、含有 would/ should/had l的时候，常用缩写形式如：
I'd=I would/should/had；You'd=You would/should/had；We'd=We would/should/had。
