1. when 的用法
when“当……时候”, 从句用的最多的是一般过去时，而主句的时态没有限制,根据具体情况而定。
When she came into my room, I was just reading a book. 她走进我房间时，我正在看书。
When I got to the airport, the guests had left. 当我赶到飞机场时，客人们已经离开了。
“当……时候”
She was about to go out for a walk when her uncle came.她正打算出去散步，就在那时她叔叔来了。She was cooking when someone knocked at the door.她正在做饭，就在这时有人敲门。
就在那时 at that time