when是中学英语中一个非常活跃的词，其意义丰富，用法灵活，不易被大家掌握。高考试题经常涉及对when的考查。请看2004年北京春季高考单项填空第26题：
We were swimming in the lake ______ suddenly the storm started.
A. when B. while
C. until D. before
此题考查的就是when的用法，从语境可知，该空表示“正在这时”，答案选A。下面结合高考试题对when的用法作一归纳，希望对同学们有所帮助。
一、when可以用作从属连词，引导时间状语从句，意为“当……的时候”，从句的谓语动词可以是延续性的（此时相当于while）,也可以是非延续性的。其引导的时间状语从句通常用现在时表示将来
[真题再现]
1. —He was nearly drowned once.
—When was ____?
—______ was in 1998 when he was in middle school.(2002北京春招)
A. that; It B. this; This
C. this; It D. that; This
2. The reporter said that the UFO_______ east to west when he saw it.(NMET2000)
A. was traveling B. traveled
C. had been traveling D. was to travel
3. —Can I join your club, Dad?
—You can when you _____ a bit older.(NMET1994)
A. get B. will
C. are getting D. will have got
４． Ｗｈｅｎ ｔｈｅ ｏｌｄ ｍａｎ ＿＿＿＿＿＿＿ ｔｏ ｗａｌｋ ｂａｃｋ ｔｏ ｈｉｓ ｈｏｕｓｅ， ｔｈｅ ｓｕｎ ＿＿＿＿＿＿ ｉｔｓｅｌｆ ｂｅｈｉｎｄ ｔｈｅ ｍｏｕｎｔａｉｎ．（２００５湖北卷）
Ａ． ｓｔａｒｔｅｄ； ｈａｄ ａｌｒｅａｄｙ ｈｉｄｄｅｎ
Ｂ． ｈａｄ ｓｔａｒｔｅｄ； ｈａｄ ａｌｒｅａｄｙ ｈｉｄｄｅｎ
Ｃ． ｈａｄ ｓｔａｒｔｅｄ； ｗａｓ ｈｉｄｉｎｇ
Ｄ． ｗａｓ ｓｔａｒｔｉｎｇ； ｈｉｄ
５． Ｉ ＿＿＿＿＿＿＿ ｈａｖｅ ｂｅｅｎ ｍｏｒｅ ｔｈａｎ ｓｉｘ ｙｅａｒｓ ｏｌｄ ｗｈｅｎ ｔｈｅ ａｃｃｉｄｅｎｔ ｈａｐｐｅｎｅｄ． （２００５北京卷）
Ａ． ｓｈｏｕｌｄｎ’ｔ Ｂ． ｃｏｕｌｄｎ’ｔ
Ｃ． ｍｕｓｔｎ’ｔ Ｄ． ｎｅｅｄｎ’ｔ
ｋｅｙ AAAAＢ
二、when可以引导时间状语从句，意为“一……就……”，相当于as soon as
例 I’ll tell her about it when she comes back. 她一回来我就把这件事告诉她。
We will stand up when the teacher comes into the classroom. 老师一走进教室我们就起立。
We will start when the team leader comes.队长一来我们就出发。
三、when可以作“如果；要是”解，引导条件状语从句，相当于if。但when作“如果；要是”解通常表示对某事是否会发生有把握,if 则表示对某事是否会发生没有把握
例 How can you hope for mercy when you show none? 如果你不宽恕别人，你怎么能希望得到别人的宽恕呢？
Let me know when you need money. 如果你需要钱就告诉我。（含有I’m sure you’ll need money之意）
Let me know if you need money.(含有perhaps you’ll need money之意)
[真题再现]
Don’t be afraid of asking for help ______ it is needed.(NMET2003)
A. unless B. since
C. although D. when
ｋｅｙ D
四、when可以作“既然，考虑到”解，引导原因状语从句，相当于since, now that, as或considering that
例 I can’t tell you anything when you won’t listen.既然你不想听，我就什么也不告诉你了。
How could you, when you knew that this might damage the apparatus? 既然你知道这样会损坏仪器，你怎么能这样做呢？
[真题再现]
１． Ｈｅ ｔｒａｎｓｐｌａｎｔｅｄ ｔｈｅ ｌｉｔｔｌｅ ｔｒｅｅ ｔｏ ｔｈｅ ｇａｒｄｅｎ ＿＿＿＿＿＿＿ ｉｔ ｗａｓ ｔｈｅ ｂｅｓｔ ｔｉｍｅ ｆｏｒ ｉｔ．（２００５上海卷）
Ａ． ｗｈｅｒｅ Ｂ． ｗｈｅｎ
Ｃ． ｔｈａｔ Ｄ． ｕｎｔｉｌ
２． Why do you want a new job _______ you’ve got such a good one already?(NMET1998)
A. that B. where
C. which D. when
ｋｅｙ ＢD
五、when可以引导让步状语从句，意为“虽然；然而；可是”，相当于although
例She stopped trying, when she might have succeeded next time. 她不再尝试了，虽然下次可能就会成功了。
Professor Li walks to work when he might ride. 李教授虽然有车可坐，但他还是步行去上班。
六、when可以用作等立连词，意为“就在那时/这时”（=at that/this time）,“然后”（=and then）,通常与“be about to do…”、“be doing…”或“be on the point of doing…”连用
例We were about to start, when a strong wind came up and it began to rain. 我们正要出发，这时刮起了大风，天开始下起雨来。
We hadn’t been out for long when she felt sick in the stomach. 我们出去时间不长她就感到肚子疼。
I was doing my homework when the telephone rang. 我正在做作业，这时电话铃响了。
[真题再现]
1. He was about to tell me the secret _______ someone patted him on the shoulder.(2002上海)
A. as B. until
C. while D. when
2. The students _____ busily when Miss Brown went to get a book she ______ in the office.(MET1991)
A. had written; left
B. were writing; has left
C. had written; had left
D. were writing; had left
七、when可以用作关系副词，引导定语从句。这时when在定语从句中作时间状语
[真题再现]
1. The film brought hours back to me ______I was taken good care of in that faraway village.(NMET2001)
A. until B. that
C. when D. where
2. It was an exciting moment for these football fans this year, _____ for the first time in years their team won the World Cup.(2000北京春招)
A. that B. while
C. which D. when
3. I shall never forget those days ______I lived in the countryside with the farmers, ________had a GREat effect on my life.(1994上海)
A. that; which B. when; which
C. which; that D. when; who
ｋｅｙ CDB
八、when可以用作连接副词，引导名词性从句。when在从句中既起连接作用同时也作状语
[真题再现]
1. I hate ______ when people talk with their mouths full.(NMET1998)
A. it B. that
C. these D. them
2. I remember ______ this used to be a quiet village.(ＮMET1993)
A. when B. how
C. where D. what
3. What I asked him is _______the story happened.(ＮMET1990)
A. when and what
B. which and where
C. what and where
D. when and where
九、when可以用于固定句型“hardly…when…”，意为“一……就……”
使用该句型时要注意：
① hardly 后的谓语动词用过去完成时 ,when后谓语动词要用过去时。
②hardly位于句首时, hardly 后的句子要倒装，而when引导的句子不倒装。
例 He had hardly left home when it began to rain.他刚出门就下雨了。
[真题再现]
______ got into the room,_______ the telephone rang.(ＮMET1988)
A. He hardly had; then
B. Hardly had he; when
C. He had not ; than
D. Not had he; when
ｋｅｙ B
十、when引导时间状语从句时，如果主从句主语一致或从句主语为it，且从句中的谓语部分含有be的形式，可将从句主语和be省略
[真题再现]
1. Generally speaking, ______ according to the directions, the drug has no side effect.(2003上海)
A. when taking
B. when taken
C. when to take
D. when to be taken
2. When ______, the museum will be open to the public next year.(2002上海春季)
A. completed
B. completing
C. being completed
D. to be completed
