ask somebody for help和ask somebody to help有什么区别,ask doing和ask to do的区别

ask somebody for help中的ask…for是短语，for是介词，help是名词。

ask somebody to help中的ask sb. to do sth.是一个动词结构，help是一个动词。例句：

If you don't know the way to the station, you may ask the police for help.

The teachers often ask us to help Mary to learn Chinese.