課本中不會(huì)教你的[W]713 when，while和 as
713 when，while和 as
① when，while和as都可以引導(dǎo)時(shí)間狀語從句，表示在一個(gè)時(shí)間段內(nèi)某個(gè)動(dòng)作正在發(fā)生，從句中用進(jìn)行時(shí)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(shí)，看見一些兒童在草地上玩。
Mother came in while she was crying. 她正在哭時(shí)，母親進(jìn)來了。
As I was sitting under a tree，I found a bag. 我坐在一棵樹下時(shí)，發(fā)現(xiàn)了一只包。
② when既可以表示一個(gè)時(shí)間段，也可以表示一個(gè)時(shí)間點(diǎn)；句中謂語動(dòng)詞既可以是延續(xù)性的，也可以是非延續(xù)性的；表示一個(gè)時(shí)間段時(shí)，從句謂語動(dòng)詞用進(jìn)行時(shí)態(tài)，表示一個(gè)時(shí)間點(diǎn)時(shí)，從句謂語動(dòng)詞根據(jù)具體情況用一般過去時(shí)或一般現(xiàn)在時(shí)。when從句謂語動(dòng)詞為延續(xù)性動(dòng)詞時(shí)，可同while，as換用。when從句謂語動(dòng)詞為非延續(xù)性動(dòng)詞，主句謂語動(dòng)詞為延續(xù)性動(dòng)詞時(shí)，不可同while，as替換。
When/While/As I was having supper，someone knocked at the door. 我在吃晚飯時(shí)，有人敲門。（was having supper表示一個(gè)時(shí)間段，have為延續(xù)性動(dòng)詞）
They were talking when I arrived. 我到達(dá)時(shí)他們正在談話。
It was twelve o'clock when she went to bed. 她是12點(diǎn)鐘上床睡的。（went to bed表示一個(gè)時(shí)間點(diǎn)）
提示
①when作疑問副詞用時(shí)，表示一個(gè)時(shí)間點(diǎn)，句中謂語動(dòng)詞要用一般過去時(shí)，不可用現(xiàn)在完成時(shí)。
When have you come？
When did you come？
你什么時(shí)候來的？
②如果when從句謂語是非延續(xù)性動(dòng)詞，主句謂語動(dòng)詞也是非延續(xù)性動(dòng)詞，when與as可換用，表示“正當(dāng)這時(shí)”，as在時(shí)間上更緊湊。
She entered just as/when I opened the door. 我一開門，她就進(jìn)來了。
③when還可以表示“雖然，既然”等意。
She walked when she might take a taxi. 她雖然可以坐出租車，但仍然步行。
Why don't you do it when you know you must do it？你既然知道必須做，為什么不去做呢？
③ while只能表示一個(gè)時(shí)間段，謂語動(dòng)詞只能是延續(xù)性動(dòng)詞或狀態(tài)動(dòng)詞，不能是非延續(xù)性動(dòng)詞；引導(dǎo)的從句常用進(jìn)行時(shí)態(tài)，表示一個(gè)動(dòng)作正在進(jìn)行時(shí)，另外發(fā)生了什么事。
He came in while I was reading the newspaper. 我正在讀報(bào)時(shí)他進(jìn)來了。
She was watering the flowers while I was working. 我在工作時(shí)，她在澆花。（主句和從句都用進(jìn)行時(shí)態(tài)）
What were you doing while he was watching TV？他看電視時(shí)你在干什么？
提示
①while不能表示一個(gè)時(shí)間點(diǎn)。
你離開房間時(shí)，請(qǐng)把燈關(guān)掉。
While you leave the room，please turn off the lights.
When you leave the room，please turn off the lights.
②從句謂語動(dòng)詞表示狀態(tài)時(shí)，一般要用while。
Strike while the iron is hot. 趁熱打鐵。
③while還可以表示對(duì)照或讓步。
She was dressed in red while her sister in blue. 她穿著紅衣服，而她妹妹穿著藍(lán)衣服。
While I like the colour of the coat，I don't like its shape. 我雖然喜歡這件大衣的顏色，但不喜歡它的式樣。
④ as表示兩個(gè)短促的行為、事件同時(shí)發(fā)生或交替進(jìn)行，有時(shí)表示“一邊……一邊……”。as從句中的時(shí)態(tài)可以是進(jìn)行時(shí)，也可以是一般時(shí)。
As（可用When）I opened the window，I saw the sun rising. 我打開窗戶時(shí)，看見太陽正在升起。
Just as he was speaking there came a knock at the door. 他正在說話時(shí)，傳來了敲門聲。
He looked behind as he ran. 他一邊跑一邊朝后看。
She sang as she danced. 她一邊跳一邊唱。（不可用when，while）
