问题描述：
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.
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.
问题解答：
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