什么情况下用不定式 急!

动词不定式在句子中可做任何成分,但不能做谓语.
(1）动词不定式做主语
eg:To swim with you was very enjoyable.
注：动名词和动词不定式均可以做主语和表语,但动名词表示经常性的,抽象的动作,而不定式表示特指的一次性的具体的动作.
(2）动词不定式做表语
eg:Her wish is to become a teacher.
注：表语为不定式时,主语也是不定式；当主语部分含有动词do时,做表语的不定式可以省略to.
eg:To see is to believe.
All you have to do is (to) study hard.
(3) 不定式做宾语
以下及物动词后常用动词不定式：
want,decide,aim(目标),manage(试图),hope,ask,happen,like,love,choose,prevent(阻止),promise,continue,plan,learn,fail等.
eg:I decide to look for a job.
(4) 动词不定式做定语
动词不定式做定语往往要后置,与被修饰词之间存在动宾关系、主谓关系、同位关系和修饰关系.如果动词不定式是不及物动词,应该在动词后加上适当的介词.如果被修饰词是地点、时间、方法,通常省略动词不定式后的介词.
eg:I have something to say.(动宾关系）
I was the last one to leave here.(主谓关系）
I have no wish to quarrel with you.(同位关系）
This is the best way to do that.(修饰关系）
I have nothing to worry about.(不及物动词后加介词）
I can't find a place to play (in).(省略介词in)
注：当名词用特定的定语如：frist,last,next,only等以及形容词*修饰时,后面用不定式修饰.
eg:He is the only person to know the truth.
(5）不定式做状语
不定式做状语主要表达结果、目的和原因.
eg:He worked day and night to play for the debts.(目的状语）
He woke up to find everyone gone.(结果状语）
We laughed to hear the news.(原因状语）
注：动词不定式与only连用时,可表出乎意料的结果.
eg:He went to seaside only to be drowned.
(6) 动词不定式做补语
eg:Allow me to introduce her to you.
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