動(dòng)詞不定式,動(dòng)名詞或主語(yǔ)從句作主語(yǔ)時(shí),通常把它們放在謂語(yǔ)動(dòng)詞前,而用it作形式主語(yǔ),放到句首,常見(jiàn)句型有：
1.It + be + 形容詞+主語(yǔ)從句,如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2.It + be + 名詞詞組 + 主語(yǔ)從句,這類名詞有：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 + 過(guò)去分詞 + 主語(yǔ)從句,這類過(guò)去分詞有：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 + 不及物動(dòng)詞(seem,appear,happen等) + 主語(yǔ)從句,這類不及物動(dòng)詞有：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 + 形容詞 + for sb.）+ 動(dòng)詞不定式
這類句型常用形容詞easy,difficult,hard,important,possible,wise等作表語(yǔ),有時(shí)候?yàn)榱藦?qiáng)調(diào)不定式動(dòng)作的執(zhí)行者,常在不定式前加for sb,如：
It’s necessary for the young to master two foreign language.
It is unwise to give the children whatever they want.
這類形容詞有：easy,difficult,hard,important,possible,wise
6.It + be + 形容詞 + of sb.+ 動(dòng)詞不定式,這類形容詞常是表示心理品質(zhì),性格特征的形容詞,如：kind,nice stupid,clever,foolish,polite,impolite,silly,selfish,considerate等.某人和這些形容詞可以構(gòu)成主系表結(jié)構(gòu).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 +名詞詞組 + 動(dòng)詞不定式,如：
It is not a good habit to stay up too late.
8.It + be + 名詞或形容詞 + 動(dòng)名詞,這類名詞和形容詞常常是：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.)+ 時(shí)間（金錢）+ 動(dòng)詞不定式,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.