当代英语常常用it作形式主语，把动词不定式放在谓语之后。 It takes ten people to encircle the big tree, How long did it take you to finish the work? 动名词在句子中也可以作主语，其逻辑主语泛指人们： Seeing is believing. Saving is having, it作形式主语，把真正的主语——动名词放在句末的情况比较少，常用的结构有：it is后接no use，no good、fun等名词，如： It is no use crying over spilt milk. It is no good objecting. It is great fun sailing a boat. it is后还可接useless、nice、good、interesting等形容词。如： It is useless speaking. It is so nice sitting here with you. It is good hearing English spoken. 1：it is unwise to (),we should do one thing at a time 2:it is impossible to (),it takes time 3:it is cruel to() 4:it was terribly unfair to() It's+adj.+of sb.+to do 这个句型，注意和It's+adj.+for sb.+to do sth的区别 这两个句型一个使用of 一个使用for 使用of：形容词形容人的品质 性格特点的时候用of，比如 clever，kind，nice 使用for，形容词就是对客观事实的描述，形容词如 impossible，difficult等 0 it is necessary for us to save endangered animals . 5. In fact_____________is a hard job for the police to keep order in an important football match. A. this B. that C. there D. it 本文来源：https://www.wddqw.com/doc/74779a290366f5335a8102d276a20029bd6463e4.html