it的用法：
⑴it用作人称代词：指代前面已经提到过的动物、事物、小孩或性别/身份不明的人，但应注意：指宠物时常用he/she，指大动物（马、大象等）可以用he/she；指祖国或家乡时，常用she.
注意句子：He failed in the examine，and it worried him. He failed in the examine. It worried him.
He failed in the examine，which worried him.
⑵it用作非人称代词：常常用作句子的主语，可以指时间、距离、价格、长度、重量、日期、星期、月份、季节、气候等。

⑶it用作形式主语或形式宾语：根据句子结构的需要，人们常用it作形式主语或宾语，而把真正的主语或宾语（不定式短语、动词-ing短语或名词性从句）放在句末。

注意it作形式主语的常见结构有：
①It+ be + adj/n（for sb）to do sth.
②It +be + adj of sb to do sth.
③It +be + adj /n+ doing sth（一般限于名词fun，good，luck，joy，use，pleasure
或形容词foolish，wonderful，good，nice，interesting，useless，worth等作
表语时，才用it作形式主语，将真正的动名词主语后置）
④It +be + adj./n + that-clause.
⑤It +be+过去分词（如said，proved，known，believed，reported，thought，hoped，
decided，announced, suggested，ordered，demanded，proposed，requested，advised
等）+that-clause.注意如果表语是划线部分的词，从句应该用虚拟语气。

⑥It takes sb some time to do sth.
⑦It seems/looks as if….
⑷it用在强调句型中：
强调句型“It is/was + 被强调部分 + 其他”的用法
①这一句型可以强调除谓语、表语、定语和同位语以外的任何部分；如果被强调的部分是人，可以用who/that，如果被强调的部分是事物，只能用that。

如：It was I who/that came here late this morning. It is a book that is on the desk.
②注意该句型不能强调让步状语从句；强调原因状语从句时，只能用because，不能用
since，as等；强调not…until句型的时间状语时，要把not与until放在一起。

另外，not until放在句首时句子必须倒装。

如：It was although it was snowing that they went out. （×） I was because he had noticed them watching him that he turned away. It was not until yesterday that he finished his homework. Not until yesterday did he finish his homework？
③在这一句型中，不管强调的主语是单数还是复数，系动词只能用is或was（一般情况下，当句子时态表示现在或将来时用is，当句子的时态表示过去时用was）。

如：
It is Jim that will go to the park tomorrow. It was Lily that finished her work.
④注意这一句型变否定句、一般疑问句、特殊疑问句和反意疑问句的变法。

特殊疑问句
变强调句型时，只强调特殊疑问词，其结构为：特殊疑问词+is/was+it+that….如：
Is it farming that he likes to take up？ Who was it that/who won first in the long jump？
Why was it that he came late？ It was I that came here late this morning，wasn’t it？
⑤注意把强调句型和“it作形式主语，that引导真正主语从句的句型”区别开来。

如果
把强调句型的结构标志去掉后，句子结构和意思仍然完整的为强调句型。

如：It is natural that he doesn’t do it at all.（主语从句） It was yesterday that I met him.（强
调句型）
注意：强调动词时，不能用强调用句型，可以把助动词do，does，did用于一般现在时
和一般过去时的肯定陈述句中，与动词原形连用，以加强语气，在句中要重读。

如：I do think he is right. He did tell me that he would join us in the work.
⑸it引起的几个易混的时间句型。

①It's time…that从句，从句中谓语动词需要用虚拟语气（“should+动词原形”或动
词过去式）如：It's time that we started out/should start out.
②This/It is the first/second…(序数词) time + that 从句（从句中常用现在完成
时态）
This/It was the first/second…(序数词) time + that 从句（从句中常用过去完成
时态）It/This/That +is/was/will be +first（second…）time+that-clause.本结构中，其中的序数词也可以修饰day，month，year等名词。

如：This is the second time that she has rung you today.
③It will/won’t be + 时间段 + before从句(从句用一般现在时态)
It was + 时间段 + before 从句（从句用一般过去时态）
注意：这种句式分为两种情况：主将从现，主过从过（即主句用将来时态时，从句用一般现在时态，表示过了多长时间以后即将发生某事；如果主句为过去时态，从句也用过去时态，表示多长时间之后发生了某事。

）如：
It won’t be long before he succeeds. It was ten years before they met again.
④It is/has been + 一段时间 + since 从句（从句中常用一般过去时态或现在完成时态）
It was/had been + 一段时间 + since 从句（从句中常用一般过去时态或过去完成时态）
注意：此句型中如果since从句中谓语为延续性动词，翻译时应翻译为否定式。

如：It is five years since he (has) arrived in China.
他来中国已经五年了。

It is five years since he (has) smoked a cigar.
他戒烟已经五年了。

注意以上两句的区别。

It was five years since he (had) arrived in China.
⑤It is/was+具体时间点+when…本结构是定语从句句型，具体的时间点是先行词。

如：
It is five o'clock in the afternoon when they climb up to the top of the mountain.
（6）it的模糊用法
①it + that 可以引导宾语从句，这样的动词或词组有: see to it（确保），depend on, rely on, insist on, take（猜想、认为）。

如：
I take it that he is not fond of reading this novel.
Our teacher saw to it that our homework should be finished on time.
② like, love, appreciate, enjoy, dislike, hate it + 介词短语 + when引导的宾语从句。

如：
I hate it in the evening when you sing.
③ make it表示“成功办到、做到、赶上”。

如：
He tried to jump over the fence and finally he made it.
④ as it is用于句首，表示“事实上”，用于句末，表示“照原样”。

如：
I thought he would get better, but as it is he is getting worse. Leave the chair as it is.
⑤ as it were意为“可以说，在某种程度上”。

如：
He is, as it were, a walking dictionary.。