when，while，as 都可以表示“当……时”，用作连词，引导时间状语从句。三者都可以置于句首或句中，置于句首时，从句后要加逗号。
01 三者主要差异
① when 既可用于有延续性的动作或场景，也可以用于瞬时性的动作，可用一般时或进行时。
It was drizzling when we walked to school.
（我们走去上学时，天空正飘着细雨。）
When she entered the house, she could smell something burning.
（她走进屋子时，闻到有东西烧焦了。）
② while 引导的从句，表示有延续性的动作或场景，即一个时间段，通常用进行时。
Infants risk suffocating while (they are) sleeping on adult mattresses.
（婴儿若睡在成人的床垫上会有窒息的危险。）
③ as 表示两件事同时发生，从句可以用一般时或进行时。
Mary shed tears emotionally as she watched the sad movie.
（看这部令人难过的电影时，玛丽感动得落泪。）
Our car stalled as we were driving through the tunnel.
（我们开车经过隧道时，车子抛锚了。）
02 三者可混用的情况
当从句表示背景、条件动作正在进行时，发生了另一个事件或动作，这时三者可以混用：
Smith wrote his first poem while / when / as he was working as a porter in a saloon in New York.
（史密斯在纽约一家酒馆当搬运工时写了他的第一首诗。）
03 不可混用的情况
① 当主从句的动作都会持续一段时间时，通常会用while 和 as，不用when：
You should wear sunglasses while (you are) sailing because the harsh glare of the sun on the water can damage your eyes.
（你出海航行时一定要戴上太阳眼镜，因为水面刺眼的反射光会伤害你的眼睛。）
The car gained momentum as it went downhill.
（车往下坡开时获得动能。）
② when 和 as 可与 just 搭配，强调正好就在那个时候，while 不能：
Just as I finished eating, my brother explained to me a question.
（我刚吃完饭，我哥哥就向我解释了一个问题。）
The scientist had a sudden heart attack just when he was one step away from figuring out the secret of immortality.
（该科学家在即将解开长生不老秘密的前一刻，突然心脏病发了。）
③ when 和 while 引导的从句可以进行化简，而 as 不能：
When he didn't know how to proceed in an emergency, he would consult his father.
= When not knowing how to proceed in an emergency, he would consult his father.
（当不知道如何处理紧急状况时，他会请教他父亲。）
While I was here, I had a good time.
= While here, I had a good time.
（在这里时，我很愉快。）
④ 当主从句两个动作有先后顺序时，只能用 when，注意，这两个动作是紧接着的：
When asked if he would buy a car, John replied that he had no money.
（约翰被问到是否要买车时，他回答称他没钱。）
⑤ 表示一段时期所发生的事时，可以用 when 和 as：
As I was young, I was quite happy.
= When I was young, I was quite happy.
（我年轻时很快乐。）
好啦，你知道「当山峰没有棱角的时候」的「当」，该用when，while，还是as了吗？在评论区给出你的答案吧~
*本文例句出自《赖世雄经典英语语法》
参考：
english.cool
Cambridge Dictionary Grammar
*本文图片出自网络，如有侵权，请联系删除。
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