这里的when引导的是一个时间状语从句。
时间状语从句(adverbial clause of time)表时间，其关联词有as，after，before，once，since，till，until，when，whenever，while，as long as，as soon as，now(that)等。如：
As the twilight was beginning to fade, we heard the sound of a carriage. 当曙光开始消退时，我们听到了一架马车的声响。
Soon after Margaret returned, the child vomited. 玛格丽特回来不久，这孩子就呕吐了。
Sometimes it gets out before I can stop it. 有时我止不住，它就蹦了出来。
Once you’ve finished, go to bed. 你干完了再去睡觉。
Since I was a child I have lived in England. 我从小就住在英格兰。
Wait until you’re called. 等着叫你吧。
When she alighted from the train there was a nearly full moon. 当她下火车时，月儿快圆了。
While I was saying goodbye to the rest of the guests Isabel took Sophie aside. 我正在送其他客人时，伊莎贝尔把索菲拉到一边。
Now that you have come you may as well stay. 既然你已经来了，就待下来吧。
You can borrow it as long as you’re not careless with it. 你可借去用，只要你不乱用它就行。
As soon as I went in, Katherine cried out with pleasure. 我一走进去，凯瑟琳就高兴地叫起来。
as，when，while，虽都表时间，但是有区别的。as多用于口语，强调“同一时间”或“一先一后”。如：
As I was going out, it began to rain. 当我出门时，开始下雨了。(as强调两个动作紧接着发生。不强调开始下雨的特定时间，故须用as而不用when)
as有时还有“随着”的含义。如：
(23)As spring warms the good earth, all flowers begin to bloom. 随着春回大地，百花开始绽放。(句中的as也不可代之以when)
when则强调“特定时间”。如：
(24)When he was eating his breakfast, he heard the door bell ring. 当他正用早餐时，听到门铃响了。(as亦可表“特定时间”，在这个意义上二者可互换，故本句中的when可代之以as)
while也表同一时间，其所表的时间不是一点，而是一段。如将上句的when改为while，while从句即强调“他吃早餐”的过程。
一些表时间的副词和短语亦可引导状语从句。如：
(25)Directly he was out of sight of her he wanted to see her. 他一看不到她就想见她。(副词directly引导时间状语从句，directly=as soon as)
(26)I didn’t wait a moment, but came immediately you called. 我一刻也没等待，你的电话一到我就来了。(副词immediately引导时间状语从句，immediately=as soon as)
(27)We’ll leave the minute you’re ready. 你准备好了我们就走。(短语the minute引导时间状语从句)
(28)The day he returned home, his father was already dead. 他回家的那一天，他的父亲已经死了。(短语the day引导时间状语从句)
(29)Next time you come, please bring your composition. 你下次来，请把你的作文带来。(短语next time引导时间状语从句)
(30)Every time I listen to your advice, I get into trouble. 每次我按你的意见办事，总是出麻烦。(短语every time引导时间状语从句)
时间状语从句中有些成分有时可省略。如：
While flying over the Channel, the pilot saw what he thought to be a meteorite. 当这位飞行员飞过英格利海峡时，他以为他看到的是一颗殒星。(while之后省去he was)
Metals expand when heated and contract when cooled. 金属热胀冷缩。(两个when之后都省去they are)
Complete your work as soon as possible. 要把你的工作尽快完成。(as soon as后省去it is)
希望我能帮助你解疑释惑。
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