1 when, while和as 都可以引導狀語從句,表示在一個時間段內(nèi)某個動作正在發(fā)生,從句中用進行時態(tài)。
When I was walking along the road, I saw some children playing on the grass. 我沿著馬路向前走時,看見一些兒童在草地上玩。
Mother came in while she was crying. 她正在哭時,母親進來了。
As I was sitting under a tree, I found a bag. 我坐在一棵樹下時,發(fā)現(xiàn)了一只包。
2 when 既可以表示一個時間段,也可以表示一個時間點(一瞬間);表示一個時間段時,從句謂語動詞用進行時態(tài),表示一個時間點時,從句謂語動詞根據(jù)具體情況用一般過去時或一般現(xiàn)在時。when 引導的時間狀語從句里的謂語動詞,既可以是延續(xù)性的,也可以是非延續(xù)性的。
When/While/As I was having supper , someone knocked at the door. 我在吃晚飯時,有人敲門。(was having supper表示一個時間段 )
It was twelve o'clock when she went to bed . 她是12點鐘上床的。(went to bed表示一個時間點 )
提示
when作疑問副詞用時,表示一個時間點,句中謂語動詞要用一般過去時,不可用現(xiàn)在完成時。
When did you come?(正)你什么時候來的?
When have you come? (誤)
3 while 表示一個時間段,引導的從句常用進行時態(tài),表示一個動作正在進行時,另外發(fā)生了什么事。要表示兩個動作同時進行或兩種情況對比時,常用while 。
He came in while I was reading the newspaper. 我正在讀報時他進來了。
She was watering the flowers while I was working. 我在工作時,她在澆花。(主句和從句都用進行時態(tài) )
What were you doing while he was watching TV?他看電視時你在干什么?
Strike while the iron is hot. 趁熱打鐵。
He fell asleep while listening. 他聽著聽著睡著了。
提示
①while不能表示一個時間點。
你離開房間時,請把燈關(guān)掉。
When you leave the room, please turn off the lights. (正)
While you leave the room, please turn off the lights. (誤)
②while可用作名詞,意為“一會兒,一段時間”,常同a連用。
After a while they went home. 過了一會兒,他們回家了。
Please wait for a while . 請等一會兒。
4 as 表示兩個短促行為、事件同時發(fā)生或交替進行,有時表示“一邊……一邊……”。
As/When I opened the window, I saw the sun rising. 我打開窗時,看見太陽正在升起。
He looked behind as he ran. 他一邊跑一邊朝后看。
She sings as she dances. 她一邊跳一邊唱。(不可用when, while )
I saw her as she was getting off the bus.她下公共汽車時我看見了她。
5 as soon as 意為“一……就”,引導時間狀語從句。注意,as soon as 所引導的從句要用一般現(xiàn)在時代替一般將來時,而其主句常用一般將來時。
We'll go home as soon as the rain stops. 雨一停我們就回家去。
I'll tell her the news as soon as I see her. 我一看到她就告訴她這個計劃。