用beforeA事先发生 。如 We waited for a long time before he came.我们等了很久他才来。
He rushed out before I could say a word.还没等我说话他就冲了出去。
I had finished my homework before my mother came back.在妈妈回来之前我就完成了作业。
用when时要看句型。 be about to do ,,,when,,, 正在做某事这时另一件事发生了。如
I was about to leave when he came.我正要走这时他来了。
had done sth when ,,, 刚做完某事这时另一件事发生了。如
I had finished homework when the power was off.我刚刚完成作业电就停了。
be +doing sth when ....正在做，，这时另一件事发生了
I was driving along the road when an accident happened.我正在路上开着车这时发生了交通事故。
,,,when,,,表示前面的事发生在后面的事之中。如
The teacher came in when we were talking and laughing happily. 我们正高兴地说着笑着的时候老师走了进来。
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