it什么时候做形式主语 it做形式主语时后的动词用不定式还是ing

it什么时候做形式主语 it做形式主语时后的动词用不定式还是ing

当不定式短语、动名词短语和从句作主语时,主语部分的长度总是超过谓语部分.在这种情况下,如果仍旧把主语放在谓语动词前,句子就会显得头重脚轻.为了调节句子的平衡,于是就把主语放在谓语部分的后面,原主语空缺的位置由形式主语it 充当“傀儡”,而真正的主语则在“后宫垂帘听政”.
 
形式主语用于三种情况.
 
　　1、如果表语是表示“好处”或“用处”的形容词或名词时,要用动名词短语作主语.常用的形容词有 good,helpful,helpless,useful,useless 等,名词有 no good,no use,no help 等.如：
 
It is helpful having you in the room when major news is expected.等待重大消息时最好有你们在场.
It's nouse praying for the weather to change.祈祷天气转变是徒劳的.
　　2.如果表语是其他形容词或名词,则用不定式短语作主语.其结构是“It + 系动词be +表语 + (for sb.) to do sth”.如：
 
It is important (for sb.) to do sth.（某人）做某事很重要.
It is necessary for sb.to do sth.某人）做某事必要的
It was unfair to (for sb.) to do sth.（某人）做某事是不公平的.
It was an effort (for sb.) to do sth..（某人）做某事是件费劲的事.
It mustbe a good job (for sb.) to do sth..（某人）做某事是件好事.
 
　　除此之外,如果表示“付出时间、金钱或精力等做某事”,也要用不定式短语作主语,其结构是“It + take/cost + sb.+ time/money/effort + to do sth”.如：
 
It took/cost him two hours to do his homework.做作业花了他两个小时.
It took/cost me over $ 6 to buy the suit.买那件衣服花掉我6英镑.
It took him a hard work to pay off the debt.还清这笔债务花费了他辛勤劳动.
 
　　【注：如果表语是 worth while,既可以用不定式也可以用动名词作主语.例如 It is worth while to discuss [discussing] the problem.这个问题值得讨论一下.】
 
　　3、“It + 系动词be +表语 + for sb.to do sth”这种结构也可以改用主语从句,结构为“It + 系动词be +表语 + 主语从句”.如果强调“某人应该做某事”是说话人的个人看法,则从句需要用虚拟语气（should常常省略）.如：
 
It is important that he (should) complete the task in time.他及时完成该项任务是很重要的.
It was necessary that they (should) come earlier.他们很有必要提早一些来.