can't, cannot, can not

can’t 是 cannot 的缩写形式，但正式场合下，cannot 比 can’t 的语气更强烈，在官方文件和正式文体中不能用缩写，如can’t。(雅思作文也不能用缩写)

The main difference between “cannot” and “can not” is that the first
term deals with impossibility and the second handles choice.

http://www.360doc.com/content/17/0625/13/8150607_666401968.shtml
https://www.sohu.com/a/206716356_100065457
https://zhidao.baidu.com/question/490908447488130972.html