can't but、can't help but、do nothing but有什么区别?

can't but do 不得不 :have to do sth.
eg,He can't but stop his work.他不得不停止工作.
can't help but to do = can't help doing 禁不住:unable to change one's behaviour or feelings
eg.We can't help but to laugh at him.= We can't help laughing at him.我们禁不住嘲笑起他来.
do nothing but 只（做某事）...nothing but = only
eg.He can do nothing but complain.他只能抱怨.
He is doing nothing but wandering about.他无所事事,只是在闲逛.