it用法难点辨析
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it可以说是英语中使用频率很高的词，也是同学们做单选题、改错题,甚至阅读、写作时经常犯错的难点所在。it的用法究竟难在哪儿·
难点一：it，one和that的用法区别
我们都知道，it具有指代作用，可以指代前后文提到的人、物、事。与it同样具有指代作用的还有one和that，它们与it的区别在于——
it是特指，而one是泛指。
This dictionary is the one needed by every student of English， but I can’t afford it. （特指）
Every student of English needs a good dictionary, but I can not afford one.（泛指）
it和that指代上文提到过的事物时，有时可以通用。但表示事物的对比时，如果对比的是同一个人或物在不同时间所处的状态，应该用it指代；如果对比的是两个不同的人、事物或地点，应该用that指代。
You have saved my life. I shall never forget it/that. （指代上文提到的“救我”这件事，用it或that均可）
The weather in Beijing is warmer than that in Alaska. （句意为“北京的气候比阿拉斯加温暖”，是不同地方气候的对比，用that指代）
Life is much easier than it used to be. （句意为“生活比过去容易多了”，是同一事物不同时间的状态的比较，用it指代）
当that指代上文提到过的名词时，后面可以跟介词短语或句子作后置的修饰语，it则不可以。
Their problem is somewhat similar to that which they faced many years ago. （指代前文提到的“问题”且后跟一个定语从句，应该用that而非it）
Life today is much better than that in old days. （指代前文提到的“生活”且后跟一个介词短语，应该用that而非it）
难点二：it领起的强调句与定语从句的区别
去掉It be... that/who(m) 这一表示强调的结构，剩下的部分可以构成一个完整无缺的句子，便是强调句；反之，则为定语从句。
It is ten years ________Miss Green returned to Canada.
A. that B. when C. since D. as
这道题应该选C，句意为“格林小姐回到加拿大已经10年了”。如果把它当做强调句填入that，那么去掉It is... that的强调结构后只剩下ten years Miss Green returned to Canada，不成为一个句子，所以这不是强调句。
难点三：与it有关的一些句型的时态选择
除了强调句以外，我们还能接触到不少与it有关的句型，它们中的一些在谓语动词的时态选择上让不少同学感到头疼。
句型It is/has been + 时间段 + since...
主句的时态可以用一般现在时或现在完成时，从句的时态常用一般过去时，且谓语动词是瞬间动词。
It is five months since I arrived in New York. 我到纽约已经5个月了。
It has been five months since I left New York. 我离开纽约已经5个月了。
句型It was/will be + 时间段 + before...
主句的时态是一般将来时，从句的时态用一般现在时表示将来。
主句的时态是一般过去时，从句的时态用一般过去时。
It will be several years before we meet again. 过几年我们才能再见面。
It was not long before they set out for the front. 他们不久就上前线了。
句型It/This is/was/will be the first/second time （that）...
主句的时态是一般现在时，从句的时态用现在完成时。
主句的时态是一般过去时，从句的时态用过去完成时。
主句的时态是一般将来时，从句的时态用现在完成时表示将来完成。
It is the first time that I have been here. 这是我第一次来这里。
This was the second time he had seen the film. 这是他第二次看这部电影。
It will be the first time that they have visited the West Lake. 这将是他们第一次游览西湖。
【练一练】
1. To tell you the truth, the accident and the damage ________ resulted in frightened me so much that I almost gave up driving ever since.
A. what B. one C. it D. that
2. —He got his first book published. It turned out to be a bestseller.
—When was ________·
— was in 2000 when he was still in college.
A. that; This B. this; It C. it; This D. that; It
3. The information on the Internet gets around much more rapidly than ________ in the newspaper.
A. it B. those C. one D. that
4. We have been looking at houses but haven’t found________we like yet.
A. one B. ones C. it D. them
5. It has been two years _______ he ________ the army.
A. before; joins B. that; joins C. since; joined D. when; joined
【参考答案】
1 C （it代替前文提到的“事故”，it resulted in是省略了引导词that的定语从句，修饰damage）
2 D （第一空用that指代“书出版很畅销”这件事；第二空是强调句型，完整的句子为It was in 2000 when he was still in college that he got his first book published）
3 D （指代前面提到的information，且后面有介词短语in the newspaper，只能用that而不用it）
4 A （句意为“我们一直在找房子，但还没找到喜爱的”。根据句意此处应是泛指且为单数，所以选one）
5 C （句意为“他加入部队已有两年”。在It is/has been + 时间段 + since ...的句型中，主句是现在完成时，从句采用一般过去时）
Key to Why Can’t We Bear the Sound of Nails on a Blackboard： A
Key to Find Your Lonely Planet：Southeast Asia.