不能做主语,但是在倒装句中介词短语可以处于主语位置,但他不是主语
e.f On the top of the mountain stands a temple ,in the temple lives an old monk .
句子中的主语分别是the moutain and an old monk,
介词短语可以做宾语
e.f An apple is on the table .这是简单句主语（an apple）+谓语(is)++宾语(on the table )
I ask him to help me.句子中"to help me "做宾语补足语,而不是宾语.