英语中的条件句分为真实条件句和非真实条件句。
If it rains tomorrow, I will stay at home.
They said they would let us know if they heard any news about him.
【详解】
详解 | 例句 | |
时态 | ①如果主句用的是一般将来时，if 引导的条件状语从句常用一般现在时。 ②如果主句是祈使句，从句也常用一般现在时。 ③如果主句含有情态动词，从句一般也用一般现在时。 | ● I will visit my grandmother if I am free. ● Let’s go shopping if you are not busy. ● You must be careful if you go swimming in the river. |
语序 | if引导的条件状语从句可以放在主句前面，常用逗号隔开；也可以放在主句的后面。 | If you don’t hurry, you will be late. = You will be late if you don’t hurry. |
转换 | ①“祈使句+and (or)+陈述句”在意义上相当于一个含有条件状语从句的复合句。 ②“if+否定句”相当于“unless+肯定句”。 | Hurry up, or you’ll be late. = If you don’t hurry up, you’ll be late. Don’t come in if I don’t call you. = Don’t come in unless I call you. |
注：当连词if作“是否”讲时，if引导的从句的时态要根据具体情况而定。
【中考链接】
( ) 1. —I hear Jack Chan will come to Laiwu next week. （2010山东莱芜）
—Really? _______ he comes, my younger sister will be very happy.
A. If B. Until C. Unless D. Before
( ) 2. I don’t know if it ______ tomorrow. If it ______, we won’t go on a picnic.
（2010 四川内江）
A. rains; rains B. will rain; rains C. will rain; will rain
( ) 3. We will have no water to drink _______ we don’t protect the earth. （2010上海）
A. until B. before C. though D. if
【巩固练习】
Ⅰ. 单项选择
( ) 1. We will stay at home if my aunt ______ to visit us tomorrow.
A. come B. comes C. will come D. is coming
( ) 2. —Do you know if he ______ to play basketball with us?
—I think he will come if he ______ free tomorrow.
A. comes; is B. comes; will be
C. will come; is D. will come; will be
( ) 3. You will keep healthy ______ you do more exercise, such as running and swimming.
A. if B. how C. before D. where
Ⅱ. 根据汉语意思完成下列句子，每空一词。
4. 如果明天不下雨的话，我们将去动物园。
We will go to the zoo ______ ______ ______ ______ tomorrow.
5. 如果你完成了作业，我们就去下象棋。
______ ______ ______ ______ ______, let’s go to play chess.
6. 如果人人都能保护环境，世界将会变得更美好。
If everyone protects the environment, the world ______ ______ ______ ______.
Key：
二、非真实条件句: 也叫虚拟条件句，表示与现实情况相反的假设情况。主句和从句中的谓语动词都需要使用虚拟语气。在表示与事实相反的虚拟语气时，谓语动词有三种形式。其具体形式如下：
时间概念 条件从句中的谓语动词 主句中的谓语动词
与现在事实相反 did/were would(could/should/ might)+do
与过去事实相反 had done/been would(could/should/might)+have done
与将来事实相反 did/were would(could/should/ might)+ do
should do
were to do
注意：1. 上述主句的谓语中，would表示结果，might表示可能性，could表示能力、允许或可能性，should主要用于第一人称后。
2. 条件从句的谓语动词为be时，不管其主语是单数还是复数通常用were。例如：
If he were here now, we could ask him about it. (与现在事实相反)
3. 混合条件句。当条件状语从句表示的行为与主句表示的行为所发生的时间不一致时，动词的形式要根据它所表示的时间做出相应的调整。例如：
If he had followed the doctor’s advice then, he would be quite all right now.
（从句表示过去时间，主句表示现在时间）
4. (省if )倒装的虚拟条件句。 虚拟条件句的从句部分如果含有were, should或had, 可将if省略，把were, should或had 移到从句主语前形成倒装。例如：
Had I seen him then, I would have been very happy.
5. If it weren’t for…和If it hadn’t been for…是两个很常用的虚拟语气句型，其意为“若不是（有），要不是有”。前者谈论现在的情况，后者谈论过去的情况。
【即学即练】
一、用动词的适当形式填空。
1. — What will you do if it _________ (rain) tomorrow?
— We have to carry it on, since we’ve got everything ready.
2. She told me that she would not go with me if it _________ (rain) the next day.
3. If I _________ (be) you, I would plant some trees round the house.
4. If I had been you then, I _________ (not quarrel) with him.
5. If the United States had built more homes for poor people in 1955，the housing problems now in some parts of this country _________ (not be) so serious.
虚拟与现实二、选择填空
1. — John went to the hospital alone.
— If he _________ me about it, I would have gone with him.
A. should tell B. tells
C. told D. had told
2. If we _________ the other road, we might have arrived here in time for the meeting.
A. take B. had taken
C. took D. have taken
3. If he _________ my advice, he wouldn’t have lost his job.
A. followed B. should follow
C. had followed D. would follow
4. Had I known about this computer program, a huge amount of time and energy _________.
A. would have been saved
B. had been saved
C. will be saved
D. was saved
5. _________ for the free tickets, I would not have gone to the films so often.
A. If it is not B. Were it not
C. Had it not been D. If they were not
6. — The weather has been very hot and dry.
— Yes. If it had rained even a drop, things would be much better now! And my
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