It作形式主语常见句型
动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：
1. It + be + 形容词+主语从句，如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2. It + be + 名词词组 + 主语从句，这类名词有：a fact, a pity, a shame, an honour，a question等，如：
It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
3. It + be + 过去分词 + 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等， 如：
It is reported that 16 people were killed in the earthquake.
It is not decided who will give the operation to the patient.
4. It + 不及物动词(seem, appear, happen等) + 主语从句, 这类不及物动词有：appear, happen, seem等， 如：
It seemed that he didn’t tell the truth.
It happened that I was out when he called.
5. It + be + 形容词 + for sb.）+ 动词不定式
这类句型常用形容词easy, difficult, hard, important, possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加for sb，如：
reportedIt’s necessary for the young to master two foreign language.
It is unwise to give the children whatever they want.
这类形容词有：easy, difficult, hard, important, possible，wise
6. It + be + 形容词 + of sb. + 动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind, nice stupid, clever, foolish, polite, impolite, silly, selfish, considerate等。某人和这些形容词可以构成主系表结构。如：
It’s very kind of you to help me with the work.
=You are kind to help me.
It seemed selfish of him not to share his dictionary with others.
=He seemed selfish not to share his dictionary with others.
7. It + be +名词词组 + 动词不定式，如：
It is not a good habit to stay up too late.
8. It + be + 名词或形容词 + 动名词，这类名词和形容词常常是：good, no good, no use, a waste of, useless, senseless等，如：
It’s a waste of time talking to her any more.
It is no use arguing about the matter with him.
9. It + take ( sb. )+ 时间（金钱）+ 动词不定式，如：
It took the workers almost three years to finish building the dam.
It will take a whole day to get to the top of the mountain on foot.