ask for:意为“请求(得到某物)；寻找(某人)”.例如：She asked her father for advice.她向父亲征询意见.Has anyone asked for me at the hotel?旅馆里有没有人找我?
ask for 可以构成许多搭配,即：要求说明原因(ask for reasons);询求建议(ask for advice);请求许可离开(ask for leave);征求意见(ask for opinions).
Ask是提问,询问,要求的意思,如：
Let me ask you some questions.
If don”t know how to do it,then ask me.
要求别人做某事,与动词不定式搭配ask sb to do sth.