选 until
Could you wait until I get the book back.
意思是：你能等我一下直到我把书取回来吗？
before的用法：
before可以作为连词，引导时间状语从句。它的用法灵活、译法多变。
1．before作“还没来的及……就……”，“未……就……”，“……就……”或“……便…”解时，强调主句或从句谓语动词表示的动作发生得过早或过快。例如：
He announced the ending of the meeting before I could get in a word.
没等我插上一句话，他就宣布散会了。
Someone called me up in the middle of the night, but they hung up before I could answer the phone.
有人半夜给我电话，但我还未来的及接他就把电话挂了。
[典型例题（加下划线的为正确答案）
① －Why didn’t you tell me him about the meeting?
－He rushed out of the room ____ I could say a word.
A. before B. until C. when D. after
② －He made a mistake, but then he corrected the situation ____ it got worse.
A. until B. when C. before D. as
2．before作“……才…”解时，强调从句谓语动词表示的动作发生得太慢或太晚。例如：
The bus driver almost knocked me down before he saw me.
那位公交车司机差一点把我撞倒，他才看见我。
The fire lasted about four hours before the firefighters were able to control it.
大火持续了大约四个小时，消防员才把火控制住。
[典型例题（加下划线的为正确答案）
③ －Several weeks had gone by ____ I realized the painting was missing.
A. before B. since C. when D. after
④ －The American Civil War lasted four years ____ the North won in the end.
A. after B. when C. before D. then
3．before可译为“趁……”、“以免”、“以防”等。例如：
I’ll do it before I forget it.
趁还没有忘，我现在就把这事做了。
We’d better finish the work in time before the boss scolds us.
我们最好按时完成工作，免得挨老板骂。
4．before可译为“（宁可……而）不愿”。例如：
He’d die before he apologizes to them.
他宁死也不向他们道歉。
I’d give up the plan before I turn to him for help.
我宁可放弃计划也不向他们求助。
5．注意“It + be + 一段时间 + before从句”中的主句和从句的时态。
该句势中，主句和从句的时态有两种情况：如果主句用一般将来时，则从句用一般现在时，意为“多长时间以后某事即将发生”；如果主句用过去时，从句也用过去时，意为“多长时间以后某事才发生”。
[典型例题（加下划线的为正确答案）
⑤ －How long do you think it will be ____ China sends a manned spaceship to the moon?
－Perhaps two or three years.
A. before B. that C. when D. until
⑥ －It was some time ____ we realizes the truth.
A. since B. when C. before D. then
until的用法：
介词until
作介词的until 的英文解释是up to a specified time (直到某一时刻)或up to the time of a specified event (直到发生某事)。但注意观察下列句子，我们还可留意到until 在具体运用中的细微之处。
1. 用于肯定句中：
1) It may last until Friday. 这可能要延续到星期五。
2) He will be working until 5 o'clock. 他将一直工作到五点钟。
3) She was a bank clerk until the war, when she trained as a nurse. 她战前是个银行职员，战时受训当了护士。
不难看出until 用于肯定句中，表示句子的动作一直持续到until短语所表示的时间为止，即表示动作的终点。一般可译为“直到……时(为止)”或“在……以前”。在这种用法中，句子的谓语动词必须是持续动词(非瞬间动词)，如live, wait , last, love, like, stay, work, continue 等。
2. 用于否定句中：
1) The secret was never told until after the old man's death. 这个秘密在老人去世后才说出来。
2) Don't open it until your birthday. 等到你过生日那天再打开。
3) She didn't sleep until eight. 她八点钟才睡觉。
不难看出until用于否定句中，表示句子的动作直到until短语所表示的时间才开始发生，即表示动作的起点。一般译为“直到……才”或“直到……之前(……还不)” 。
二、 连词until
以同样的方式分析作连词的until的用法。作连词用的until的英文释义是up to the time when(直到……时为止)。
1. 用于肯定句中：
1) Heat can be conducted from a hot body to a cooler one until both are at the same temperature.热可以从一个热的物体传到一个较冷的物体，直到两者温度相同为止。
2) Continue in this direction until you see a sign. 一直朝着这个方向走就会看见指示牌了。
3) He ran until he was breathless. 他一直跑到气喘吁吁才停下。
作连词的until用于肯定句中，表示主句的动作一直持续到从句动作发生或状态出现时为止。一般可译为“……直到……为止”。在这种用法中，主句的谓语动词必须是持续动词(非瞬间动词)，如live, wait , last, love, like, stay, work, continue 等。
2. 用于否定句中：
1) I hadn't realized she was foreign until she spoke. 她不说话我还一直不知道她是个外国人。
2) I won't stop shouting until you let me go. 你不放我，我就一直喊叫。
3) Among the great mathematicians there are about as many who showed mathematical talents in childhood as there are those who showed none at all until they are older. 在伟大的数学家中，童年就显露数学才华的人和直到相当年龄才显露才华的人差不多一样多。
until与before的区别
1.可直接换用的情况:在until能使用的句型中,即主句含有否定式终止性谓语动词或肯定式延续性动词,可与before直接替换,意义差别不大。如:
①The students won't go home until / before they finish their homework.学生们要先完成作业才能回家。
②The woman worked in the factory until/before 1990. 1990年以前这妇女在这家工厂上班。
2.不可换用的情况:当主句含有肯定式终止性谓语动词时,只能用before不能用until,且before常意为"没来得及……"或"……才……"等。如:
③Einstein almost knocked me down before he saw me.爱因斯坦差不多将我撞倒才看见了我。
④He had measured me before I could get in a word.我还没来得及插话,他就把我的尺寸量好了。
作连词的until用于否定句中,表示主句的动作在从句的动作发生之前尚未发生，或者说主句的动作在从句的动作发生之后才开始。一般可译为“直到……才”。
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