tom slipped into the house when no one was noticing这句话为什么同过去进行时?

题目

tom slipped into the house when no one was noticing这句话为什么同过去进行时?
是因为when的关系吗?
那什么时候用进行时?什么时候不用?
为什么这句话不用呢?he had walked three or four miles when he saw a cart,halg-full of hay,by the side of the road.

答案

Tom slipped into the house when no one was noticing,因为是在当时没有人看见的这个时间溜进去的,说的是这个时间的事,同时发生.也正是这个原因所以才用了When.
he had walked three or four miles when he saw a cart,是当他看见马车的时候已经走了三四公里,一前一后发生的.所以是个完成时态.