Question

to 什么时候是介词，什么时候是不定式

Answer 1

非谓语动词，又叫非限定性动词，在句子中不是谓语的动词，主要包括不定式，动名词和分词(现在分词和过去分词)，动词的非谓语形式。
不定式to do是非谓语动词的一种，形式是to do 是由动词do变过来的，前面的to 不是介词而是小品词(和动词进行补充说明关系)，to后面要加动词原形。
不定式表示的是，想做，将做，未做的事强调的是目的性。
to 作为介词后面只能加名词(名词，动名词，名词性从句等)。

1)作主语: To help each other is good.
动词不定式作主语，一般可以用it 作形式主语，而将主语的动词不定式置于句末。
It is good to help each other.
2)作表语：My job is to drive them to the power station every day.

3)作宾语：
-1,作及物动词的宾语： He wishes to be a musician.
-2, 作形容词的宾语：I am determined to give up smoking.
-3,动词不定式一般不作介词的宾语，但动词不定式如有疑问词时，就可作介词的宾语，：Can you give us some advice on what to do?
4)，宾语补足语：Tell the children not to play on the street.
5),动词不定式作宾语时，如带有宾语补足语时，须用it作形式宾语
I don't think it right to do it that way.
6)作定语 ： Is this the best way to help him?
和定语用的动词不定式如果是不及物动词，不定式后面要用必要的介词，He is the man to depend on his father.
如果不定式后面修饰的名词位place, time, way, 不定式后面的介词，习惯上可以省略：The old man is looking for a quiet place to live.
(定语和宾补的区分方法：如果不定式是出现在：“动宾宾补”结构后面的话，不定式应该是做宾补了；而如果不定式是出现在“系表宾”结构后，那不定式应该是做定语了)
7)作状语 (修饰谓语动词)
-1，目的状语 ：Every morning He gets up very early to learn English.
也可以将不定式放置于句首，To learn a foreign language, one must work hard at it.
-2 They lived to see the liberation of their home town.
8)复合结构不定式：for + 名词(代词宾格) + 不定式，构成复合结构的动词不定式，for本身无意义，for 后面的名词或代词是不定式的逻辑主语，这种不定式在句中可作主语、表语、宾语、定语或状语，如：It's very importment for us to get everything ready for the harvest.
当表语用的形容词表示逻辑主语的性质或特征时，就用介词of而不用for ，引出不定式的逻辑主语，例如good, nice, kind, wise, silly, stupid, foolish, right, wrong, careless, impolite等，如：It is very kind of you to help me every day.

9）疑问词加动词不定式，疑问代词或疑问副词 加动词不定式在句子中可以做主语，表语，或宾语。: How to prevent them from swimming in the river is a problem.
10),动词不定式的否定形式，动词不定式的否定形式由 not + 动词不定式组成，
It’s foolish of you not to attend the meeting.