(1) be doing sth ---- when ---

例：He was still smiling when the door opened and his wife came in.

    他正笑着的时候门突然开了，他妻子走了进来。

(2) be about to do sth --- when ---

例：We were about to start when it began to rain. 我们刚要出发，天就开始下雨了。

(3) had not done sth --- when ---/ hardly --- when ---

例：He had not fallen asleep when the telephone rang. 他刚要入睡电话就响了。

(4) had just done ---- when ---

例：I had just gone to bed after a very hard day when the phone rang.

在劳累了一天之后我刚刚就寝，电话铃就响了。