1．It's time（for sb．）to do sth．／It's time for sth．译为“是（某人）该干……的时间了”、“到……的时候了”。如：
It's time for supper ／to have supper．
2．It takes（sb．）some time to do sth．译为“（某人）花……时间做某事”。如：
It took me a week to finish reading the book．
3．It is one's turn to do sth．表示“轮到某人做某事”。如：
It's your turn to be on duty tomorrow．
4．It is ／has been ＋时间段＋since ＋一般过去时。译为“自从……以来已过了……（时间）”。此结构可以与另一种句型进行同义句转换。如：
It has been two weeks since we met last．＝Two weeks has passed since we met last．
5．It seems ／seemed ＋that从句。译为“看起来好像……”，此结构可以转换成“seem ＋不定式”形式。如：
It seems that he is ill．＝He seems to be ill．
6．It's＋adj．＋of／forsb．＋不定式。对于这个句型究竟用of还是用for，上面许振铎老师的文章中已有论述。这里再强调一下：如果形容词是描述不定式行为者的性格、品质的，如：kind，good，nice，clever等用of；如果仅仅是描述事物的形容词，如：difficult，easy，hard， important，dangerous等则用for。如：
It was very kind of you to visit me when I was ill．
It's important for him to learn English well．
7．It's＋表语 ＋to do sth．。如：
It's a good idea to go out for a walk．＝To go out for a walk is a good idea．
8．It's＋adj．＋ that从句。如：
It's true that he may fall behind the other students．
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《高中英语语法...