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TO的两个功能
曾经有朋友问我：“I look forward to hear from you soon” 对，还是“I look forward to hearing from you soon” 对。我说后者对，前者错。他说，为什么“to”后面的动词可以有“-ing”呢？
这问题很有意思，相信其他人也会这样问。
这问题就是今天的话题了。
大家知道，“to”是不定式动词（Infinitives）的标志，如：to come, to go, to eat, to sleep, to play, to work, to sing, to take等等。
不定式动词是英语三种无限定动词（Non-finite verbs）中的一种。它的主要功能包括协助组成动词句型。例如：
1. 主语＋动词＋不定式动词／短语
(1) Helen likes to sing.
(2) The man wants to see you.
2. 主语＋动词＋宾语＋不定式动词/短语
(3) Jack advised me to come.
(4) She asked the boy to go out with his brother.
3. 主语＋动词＋宾语＋不带“to”不定式动词／短语
(5) We all heard them laugh.
(6) They saw the leaves fall down.
(7) Did you notice the old man rush to the bus stop?
(5)-(7)里的”langh, fall, rush”都是不带“to”的不定式动词或短语。
此外，当不定式动词在助动词或情态动词后面时，也是不带”to”的。例如：
(8) John will come soon.
(9) Shall I go with them?
(10) You may quit whenever you wish.
(11) We must not eat too much meat.
(12) Tony wished his friends could help him.
不管带“to”与否，不定式动词的性质是不变的。
“To”的另一个功能是当介词，性质和“at, in, on, for, with”等一样，只是形式和不定式动词的标志一样而已。正因为样貌相似，介词“to”在某些句式中常被误以为是不定式动词中的“to”而引起用法上的错讹。文章开头提到的“hear”和“hearing”之争，便是最好的例子。
须知“look forward to” 是个片语动词，这里的“to”是个介词，后面要带宾语（object）（即名词、代词或动名词），如“I look forward to your reply. / ... it. / ... hearing from you.”
下面这句情况一样，所以(13) a 错，(13) b对：
(13) a. Are you used to live in a hot country like Singapore?
(13) b. Are you used to living in a hot country like Singapore?
根据这原理，下列这些惯用语中的“to” 都是介词，不是不定式动词的标志。既然是介词，就要有宾语：
(14) Do you object to（反对）picnicking on every weekend?
(15) Many people learn English with a view to（目的是）getting a good job.
(16) A good government often devotes itself to（致力于） giving the people a good life.
(17) You must not confine yourself to（局限于）talking about your own achievements.
(18) Jason prefers working hard to（喜欢） doing nothing.
(19) Kelvin is addicted to（沉溺于） smoking.
(20) Qi is much given to（热衷）playing outdoor games.
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