While 用法1：
問：有關句子：“While he got up, he felt dizzy”。英語老師告訴我，必須將句中的while改為when，句子才是正確的。但我至今不解。 While 用法和when不是都可以引導時間副詞子句嗎？
答：雖然while 用法和when都能引導時間副詞子句，而且意思相同，但是它們在用法上有一個很重要的區别： while子句中的動詞必須是持續性的 (be, stay, wait, live等等) 而不能是瞬間性的 (get up, arrive等等)。當when子句中的動詞是持續性動詞時，就可以用 while來代替when：
- Be careful when crossing streets. 過馬路時要小心。
- ______crossing the street, he was knocked down by a car. (A. By; B. For; C. At; D. While)
請注意下面 while子句動詞的持續性：
- They arrived while I was sunbathing. 當我正在進行日光浴時·他們來了。
- We must strike while the iron is hot. 我們必須趁熱打鐵。
- While the discussion was still going on, Gerhardt came in. 當討論還在進行的時候，格哈特走了進來。
- He wrote his greatest novel while working on a freighter. 他在貨輪上工作時寫出了他那部最優秀的小說。
While 用法2 (比較when)：
問：The population density per square mile in the United States is twenty-two people, while that in Canada is two people.
上面句中的 while 用法是否可用when來代替?
答：when與while 用法上的另一個重要區别，就是 while 用法可用於表示比較(whereas)，而when卻沒有這個用法。
- Jane was dressed in brown while Mary was dressed in blue. 珍穿褐色衣服，而瑪麗卻穿藍色衣服。
- While their country has plenty of oil, ours has none. 他們國家有充足的石油，而我們國家卻沒有。
While 用法3 （比較as）:
問：I’m going to the post office.
________ you are there, can you get me some maps?
A. As
B. While
C. Because
D. If
我在考試中選了A，但答案卻是B。我還是不明白為什麼不能選A呢？後來我在《英語沙龍》1999年第11期(p51) 看到對類似上題的解答。那上面說，不能選A的原因，是因為as表示“一邊…一邊…，主子句中的動詞的動作同時進行。如：He sang as worked.他一邊工作，一邊唱歌”。難道引導時間副子句的as只有這麽一種含義嗎？
答：可以想像得出，出題者的意圖是：while 用法引導持續時間副詞，而as引導非持續時間副詞，因而不能表達主要子句動作發生的持續間背景。出題者也可能認，as只能表示“随著”、“一邊…一邊”的意思。其實，as既可以引導非持時間副詞(A)，也可以引導持時間副詞(B)：
A.
1. He came just as I reached the door. (as引導的子句表示瞬間，即時間點) 當我剛到達門口的時候，他就來了。
2. As they opened the door on the ground floor, Marie said, “Don’t light the lamps. Look!”. 當他們打開一樓的門時，瑪麗說：“別開燈。你瞧!”
B.
1. Mother was worried because little Alice was ill, especial _______ Father was away in France. (A. as; B. that; C. during; D. if)
2. Balloons and light aeroplanes can be used to pull huge signs as they fly slowly over a city. 可以用氣球和輕型飛機拖帶大型廣告在城市上空慢慢地飛翔。
3. As I was walking one day in Hyde Park, I noticed two elderly ladies. 有一天，當我在海德公園漫步的時候，我看到了兩位老年婦女。
4. As you go round the gallery look out for paintings by Tumer. (as引導的子句作持續時間副嗣，可以用when或 while來替換) 當你在美術館参觀時，去看一看特的繪畫。
5. As he stood there he saw two men enter the house. (as = while = during the time) 當他站在那裡的時候，看到兩個男人走進了那棟房子。
有專家明確指出：
“While and as suggest something continuing for a period of time.
As we were cycling along, we saw a fox.
The doorbell rang as/while I was changing.”
（“while 用法和as可表示某事繼續一段時間……”）。
據此，我們有充分的理由說，上面試題中選擇項A(as)也是同樣正確的。