1.can’t help doing sth．和can’t help but do sth 表示“不得不、禁不住做某事”,
We couldn't help laughing when we heard the funny story.
2.can’t help to do sth．表示“不能帮助做某事”,
I can't help (to) wash the dishes because I am busy preparing for the entrance exams.
而couldn't 是can't 的过去式
不明白的再问哟,请及时采纳,多谢!