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动词不定式作主语时,常用it作形式主语,而将真正的主语放在句末,避免句子头重脚轻。其结构为It+be+adj.+to do sth.
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To learn English well is useful.=It is useful to learn English well.
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如果要说明不定式表示的动作的执行者,可以在不定式前加一个for短语。其结构为It+be+adj.+for sb.+to do sth.
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It is good for you to go out for a walk.
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某些表示人的品质、特征等的形容词(如kind,good,nice,wise,clever,polite等)作表语时,不定式之前可以加一个of短语,用来说明动作的执行者。其结构为It+be+adj.+of sb.+to do sth.
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It’s not polite of him to speak to old people like that.
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两个或两个以上的动词不定式并列时,第一个不定式带to,后面的不定式通常省略to
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To go to sleep early and get up early is a good habit.
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不定式用在及物动词后作宾语,常见的及物动词有:begin,choose,continue,decide,expect,fail,forget,hate,help,hope,learn,manage,mean,need,offer,plan,prefer,pretend,promise,refuse,try,afford,agree,start,like等
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I can’t afford to buy a digital camera at the moment.
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在find,think后跟不定式作宾语时,常用it代替,而将真正的宾语放在后面
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I find it easy to read English every day.
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常接不定式作宾语补足语的动词有:want,ask,tell,allow,get,would like,advise,encourage等
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My mother encourages me to learn Japanese.
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一些使役动词和感官动词也用不定式作宾补,这时不定式要省略to。这些动词有:一感(feel)、二听(hear,listen to)、三让(let,make,have)、四看(look at,see,watch,notice)。但变被动语态时必须加上to
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I often hear Li Ming sing in the next room.
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动词help后面的不定式,既可以带to,也可以不带to
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You must help me(to)do my homework this afternoon.
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I have some clothes to wash.
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如果不定式是不及物动词,且与它所修饰的词构成逻辑上的被动关系,则不定式要加上相应的介词
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He is looking for a room to live in.We have a lot of things to talk about.
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当不定式修饰的名词是time,place或way时,不定式后面习惯上省略介词
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My grandma had no place to live(in)in the old days.
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不定式修饰 something,anything,nothing时,放在它们的后面,如果something,anything,nothing有形容词修饰,词序是“something/anything/nothing+形容词+不定式”
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I had something cold to drink.
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To be a good student,one must study hard.
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和某些形容词连用构成“be+形容词+不定式”结构,作原因状语
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We are sorry to trouble you.
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作程度或结果状语时,常与enough(表示肯定)或too(表示否定)连用
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He is old enough to go to school.He is too young to go to school.
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表示主语的“职业、职责或性质”等,通常对连系动词前面的名词进行解释说明
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His wish is to become a doctor.
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不定式常和疑问词what,which,when,where,how连用,在句中担当主语、宾语、表语等成分
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He didn't know where to go.=He didn't know where he should go.
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