to 的用法：to这个词什么时候用什么时候不用它呢?

建议你去翻一下《牛津高阶英汉双解词典》里面有to的含义及用法的全面注释
to1
KK:[]
DJ:[]
prep.
1.向,往,到
She lives in the house to the right.
她住在右边的房子里.
This road leads to Paris.
这条路通往巴黎.
2.(表示距离)离
3.(表示时间)直到,在...之前
He wrote from morning to night.
他从早到晚写东西.
4.(表示程度、范围)到,达
He was wet to the skin.
他浑身湿透了.
5.趋于;(变)成,(变)到
Things are going from bad to worse.
情形每况愈下.
6.(表示位置)对,面对
7.(表示接近、接触)在,于,紧贴着
8.(表示对象)对,向
Excessive drinking did harm to his health.
过量饮酒损害了他的健康.
9.属于,归于
I've lost the key to the house.
我把屋子的钥匙丢失了.
10.(表示比较、对比)比
This engine is superior to that one.
这台发动机比那台要好.
11.(表示比例关系)每
12.对于,关于
What is her attitude to the issue?
她对这个问题抱什么态度?
13.与...一致,按...
Modern paintings are not to my taste.
现代画不合我的口味.
14.为了,用作
We sat down to dinner at six.
我们六点钟坐下来吃饭.
15.致使
To her delight,she passed the examination.
使她高兴的是她考试及格了.
16.由于
ad.
1.向前
2.(门等)关上
She pushed the door to.
她用力把门关上.
3.恢复意识
The girl came to after a while.
过了一会儿那个女孩醒过来了.
4.(做)起来,着手
They fell to with great relish.
他们津津有味地吃了起来.
5.在近旁
to2
KK:[]
DJ:[]
prep.
1.(与原形动词一起构成不定式)
The government offered to give us financial help.
*提出在经济上给我们援助.
To gossip about others isn't right.
说闲话是不对的.
2.(用来代替不定式或不定式短语,以免重复)
You can do it this way if you care to.
如果你乐意的话,可以这样做.