介词在句子中当什么成分?介词短语呢?

介词是不能单独用的,所以,单个介词不充当任何句子成分.介词必须和名词、代词、动名词连用构成介词短语才能在句子中充当表语、定语、宾语补足语和状语等成分.如
He lives in Beijing.状语
The man in a black coat is quite strange.定语
He always keep a gun in his pocket.宾语补足语
I am in my car.表语
He suceeded in doing this.状语
I'm behind you.表语