1、 一般用when的情況，從句是短暫性動詞
1）若從句是一個短暫性動作，而主句是一個持續性動作
When he came in， I was listening to the radio.
It was raining hard when we arrived.
2）若主、從句表示的是兩個同時(或幾乎同時)發生的短暫性動作
I thought of it just when you opened your mouth.
2、 一般用while的情況，若主、從句表示兩個同時進行的持續性動作，且強調主句表示的動作延續到從句所指的整個時間。
Don't talk while you're eating.
I kept silent while he was writing.
3、 When和while可以互換的情況
He fell asleep when [while] he was reading.
I met him when [while] I was taking a walk in the park.
1，when
什麼時候，何時什麼情況下，什麼場合下(用於時間的表達方式之後)在那時其時當時（副詞）
什麼時候，何時（代詞）
在…時候，當…時，在…期間，在…之後在任何…時候（連詞）
I had an active social life when I was at college.
我在大學時社交生活豐富。
Steve was in a particularly bad mood when he got back.
史蒂夫回來時心情特別不好。
When will he be back from his holiday?
他度假什麼時候回來
I mind the time when we were in Beijing together.
我想起我們一起在北京的時光。
2，while
在…期間，當…的時候與…同時（連詞）
雖然，儘管（連詞）
(對比兩件事物)…而，…然而（連詞）
一段時間一會兒（名詞）
Flats in the downtown are expensive， while houses in the suburbs are cheap.
市中心的公寓很貴，而郊區的房子很便宜。
While I sympathize， I can't really do very much to help.
雖然我很同情，但我實在幫不了什麼忙。
Did you sneak into my room while I was asleep?
我睡着的時候你偷偷溜進我的房間了嗎
The band played the guitar while we danced.
我們跳舞時樂隊彈起了吉他。
While I was waiting at the bus stop， three buses went by in the opposite direction.
當我在公共汽車站等車時，有三輛公共汽車朝相反的方向經過。
I want to rest a while.
我想休息一會兒。
After a while， we naturally started talking about the children.
過了一會兒，我們自然而然地開始談論孩子們。
They chattered away happily for a while.
他們愉快地聊了一會兒。
Life moves pretty fast. If you don't stop and look around once in a while， you could miss it.
人生如白駒過隙。如果你不偶爾停下來四處看看，你可能會錯過許多。
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