1.介词短语既可以作状语,又可以作表语、定语和宾补.作状语时,可以前置也可以后置；作定语必须后置；作表语要放在系动词之后；作宾补要放在宾语之后：
In the office there are two desks= There are two desks in the office.
He is in the room.The man in the room is my uncle.I find her in the room.
2.状语有三个位置：句首、句中、句尾
On top of the mountain he saw a grand temple.
He stayed in the cave.He slowly stepped into the cave.