it的用法
一、用作人称代词：
1、指代事物：
— What’s this?
— It’s a cat.
2、指代人：常用于不知对方性别时，比如：询问敲门人或打电话时询问对方是谁，或者用来指代婴儿。
(1)—Who’s it?
—It’s Sam.
(2)—Who’s the baby in the picture?
—It’s my younger brother.
二、用作非人称代词：指代天气、时间、季节或距离等，此时的it无意义
(1)It’s two o’clock now.
(2)It’s very cold in spring in the northern China.
(3)It’s about 100 kilometers from our town to the big city.
三、用作替代词：it可作形式主语、形式宾语，可代替不定式、动名词或从句
(1)It is not known where she has gone.
(2)It’s bad for your eyes reading in bed.
(3)It is clever of him to work out the math problem.
四、用于强调句，构成“It+be +被强调部分that…”结构。
如：It’s in the small shop that I bought the important book.
五、在初中英语教材中，出现了许多由it引导的固定句式，现归纳如下：
(一)It's time for sb. to do sth. / It's time for sth.意为"是(某人)该干……的时间了"、"到……的时候了"。
例如： 1. It's time for supper ／to have supper.该吃晚饭了。
2. It's time for me to study.我该学习了。
(二)It takes (sb.) some time to do sth.意为"(某人)花……时间做某事"。
例如：
1. It takes twenty minutes to go to school by bike.
骑自行车上学要花二十分钟的时间。
2. It took me a week to finish reading the book.
我花了一周时间读完这本书。
(三)It is one's turn to do sth.意为"轮到某人做某事了"。
例如： It's your turn to be on duty tomorrow. 明天轮到你值日了。
(四)It is ／ has been ＋时间段＋since ＋一般过去时，意为"自从……以来已过了……(时间)。"
例如： 1. It is / has been four days since I caught a cold. 我感冒已四天了。
2. It is / has been two weeks since we met last. 自从我们上次见面以来已过了两周。
(五)It seems ／seemed ＋that从句，意为"看起来好像……"。
例如：It seemed that our team was going to win. 我们队看起来好像要赢了。
(六)It's＋表语＋to do sth.。
例如：It's a good idea to go out for a walk. 出去散步是个好主意。
(七)It's ＋adj.＋that从句。
例如：It's true that I may fall behind the other students.我真可能落在别的学生后面。
(八)It's＋adj.＋of(for)sb.＋to do sth.。