when 句型：
(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。