◇It's really kind of you to help me with my math.
相当于You are really kind to help me with my math.
1、不定式作定语与被修饰的名词构成逻辑上的动宾关系，且与句中的另一名词或代词有主谓关系时。
◇Please buy me some newspapers to read.
◇We have lots of difficulties to overcome for the time being.
2、在“be+性质形容词+不定式”结构中。
这种结构中的形容词说明不定式的特性，句子的主语是不定式的逻辑宾语。常见的此类形容词有easy,hard,difficult,comfortable,interesting,safe等。
◇This machine is very easy to operate. Any body can learn to use it in a few minutes.
这台机器操作起来很容易。任何人都能在几分钟之内学会使用它。
3、不定式与疑问词连用时
◇I don't know what to do next.
4、某些动词的不定式与be连用时。(常见有blame,seek,let等）
◇Some people felt she was to blame for this accident.
1、有些固定结构中需要省略to的不定式
◇You'd better to choose the activities which interest you.（×）
☆You'd better choose the activities which interest you.（√）
○had better (not) do sth. 最好（不要）做某事
○would rather do sth. 宁愿做某事
○Why not do sth.? 为什么不做某事
○Could/Would/Will you please (not) do sth.? 请你（不要）做某事好吗？
2、主语部分含有do的某种形式时，作表语的不定式可省略不定式to
◇All you need to do now is (to) finish the work.
☆3、在表示“除了......外”之意的介词but,except等词前，若有do的任何形式，其后的不定式要省略to；反之，则不用省略。
◇I can do everything around the house except cook.
除了做饭，家里所有的活我都能干。（前有do，后省略to）
◇I have no choice but to accept the fact.
除了接受这个事实，我别无选择。（前无do，则加to）
4、两个或多个不定式短语表示并列关系时，后面的不定式to则省略。如果多个不定式短语表示对比关系，则一般保留to。
◇He refuse to spend time with us or do as we tell him.
◇They came not to save us, but to conquer us.
小邓
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