先行词 it 作主语或宾语 (Anticipatory it as Subject or Object)

　　1． it 做形式主语代替不定式、动名词、主语从句，如：
　　　　It is easy to distinguish the two words.
　　　　It is no use talking about it any more.
　　　　It is not yet decided who is to take up the work.

　　2． it 作形式宾语代替不定式、动名词、宾语从句，如：
　　　　We found it not difficult to translate this article.
　　　　I think it useless learning a theory without practice.
　　　　We have made it clear that agriculture is the foundation of our national economy.

　　强调句中的 it (it in Emphatic Sentence)

　 　1． 强调主语，如：
　　　　It was I who (that) met Prof. Smith yesterday.
　　　　It is the computer that is doing many jobs.

　　2． 强调宾语，如：
　　　　It was Mr. White that (whom) Mary met in the park.
　　　　It is oxygen that we usually take up first in our study of chemistry.

　　3． 强调状语（时间或地点），如：
　　　　It was in 1940 that he joined the army.
　　　　It was at the school gate that I met Mr. Smith.

　　4． 强调主语补足语或宾语补足语，如：
　　　　It is liquids that all gases will become when subjected to high pressure and low temperature.
　　　　It was chairman of the department that people elected him.