"it"一词在英语中应用广泛，形式多样，充分了解其相关结构和用法，会对我们的英语学习很有帮助。

　　1. it用作人称代词

　　it用作人称代词，不仅可以指事物，也可指人。它的主要功能是指代。为避免重复，可用它来代替词、词组或整个句子。

　　①-Have you seen my dictionary?

　　-Yes, it is on my desk.

　　②-Who is there? -It is me.

　　2. it用作非人称代词

　　it用作非人称代词，其作主语时通常没有具体意义，用于表示气候、天气、温度、时间、地点、距离和环境等意义的句子中。

　　①It is ten miles from here.

　　②It is too noisy for me to study here.

　　指代时间的it还可以用在It is...since...句型中。

　　It is ten years since we got to know each other.

　　3. it用作先行代词

　　it作先行代词的用法是充当形式主语或形式宾语，后面的真正主语或真正宾语通常是不定式结构、动名词或名词性从句。

　　1)代替不定式，被替代的成分要置于句末。

　　①It is of great help to master a foreign language.

　　②It took me a week to rewrite the paper.

　　③I consider it advisable to tell her beforehand.

　　④I find it quite necessary to make some changes.

　　2)作先行代词，代替动名词。

　　it代替动名词，主要用于下面的结构中。注意，这种结构中不宜用不定式。

　　It is no good (no use, useless, senseless, dangerous, enjoyable, worthwhile) +动名词

　　It is a waste (a nuisance, hard work) +动名词

　　①It is worthwhile making another try.

　　②It's dangerous sleeping out in the woods at night.

　　③It is hard work persuading him into doing it.

　　④I find it a waste spending so much money drinking and eating.

　　3)作先行代词，代替名词性从句。

　　该结构中的名词性从句可用that, what, when等引导。

　　①It is obvious that metals in common use are very important in our life.

　　②It happened that she was not at home when I called.

　　③It is not known what caused the accident.

　　④Has it been found out who is the murderer?

　　⑤We have made it clear that she has nothing to do with the case.

　　⑥It is a mystery when they got married.

　　4. it用于强调结构中

　　1) It is +被强调部分+ that...

　　该句型中，将被强调部分放在that前，其它部分置于that之后。被强调部分可以是主语、宾语、表语或状语。如果去掉"It is...that..."结构，句意仍然完整。

　　It is from the sun that we get light and heat.

　　2) It is not until +被强调部分+ that...

　　该句型主要用于强调时间状语，译作"直到……才……"。

　　It was not until she took off her dark glasses that I realized she was a famous film star.

　　= Not until she took off her dark glasses did I realize she was a famous film star.

　　= I didn't realize she was a famous film star until she took off her dark glasses.

　　5. it的习惯用法

　　it的习惯用法较多地出现在口语或非正式文体中，这时it已失去了它的原意。

　　①Win or lose, we'll stick it out.

　　②There is nothing for it but to stand.

　　③We had a nice time of it.

　　(文/马志红；英语辅导报大学二年级版03～04学年第36期；版权归英语辅导报社所有，独家网络合作伙伴新浪教育，未经许可，不得以任何形式进行转载。)