is not缩写形式是isn't。
与not相关的否定缩写：通常在原词后加n't，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。
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。
