1. be doing/ be about to do/ had done…, when…(when：这时, 强调一个动作的突然发生)
1). I was walking along the river, when I heard a drowning boy cry for help.
2). I was about to leave when it began to rain.
3). I had just finished my test paper when the bell rang, announcing the exam was over.
2. It was (not ) + 时间段+before +一般过去时 过了一段时间就……..
