在大多数情况下，我们会看到 to 后面要接动词不定式，比如以下这两个句子（这篇文章里的所有例句都来自当代美国英语语料库，Corpus of Comtemporary American English）：

The students wanted to watch these recordings over and over.

As the Fed begins to raise rates, the equation should change.

然而，我们偶尔也会看到 to 加 doing 的形式。我想大家最常见的莫过于 look forward to doing，比如：

I look forward to working with your vibrant community.

I look forward to meeting the digitized representation of Joel.

就我这几年修改论文的经验来看，很多人都不知道 look forward to 后面应该是 doing 而不是 do。不仅中国学生，我注意到就连德国和荷兰大学的教学秘书，在发给我的电邮中, 也时常出现 look forward to do 的错误。

除了look forward to doing, 可能大家还遇到过 contribute to doing, dedicate to doing 等等，比如：

Post-Reformation developments contribute to creating a common understanding and history.

He dedicated his life to reducing the stigma of mental health.

何时用 to do 何时用 to doing 也许会让很多人感到困惑。"To do", or " to doing", that is the question. 我们当然可以把这些 to doing 的结构当作固定用法个别记忆，毕竟这样的结构也不多。但是，其实 to 后面什么时候加 do, 什么时候加 doing 是有道理可讲的。

这个道理就在于 to 有两个功能，首先 to 可以作为所谓的infinite marker，也就是标记后面的动词为非限定性动词。在 want to 和 begin to 中，to 扮演的就是这个角色。to 的另外一个功能就是作介词，就如同 for 或者 of 这样的介词 一样。而我们知道 for 或者 of 后面是加 doing的，因此这就解释了为什么当 to 作介词时后面也应该是 doing。

那么我们如何判断 to 何时是 infinite marker, 何时是介词呢？这里有一个方法。如果 to 后面可以接一个名词 (更准确的讲法是名词短语)，那么 to 这时就是介词，如果 to 后面不能接名词，那么这个 to 就是infinite marker.

比如，我们可以说 The students wanted to watch these recordings. 但是我们不能把 to 后面的 do 替换成一个名词，比如The students wanted to these recordings. 就说不通（这里的these recordings 也可以是其他任意名词）。

但是当 to 作介词时，to 后面既可以接一个名词，也可以接 doing。 本质上 -ing （学名叫gerund）就是将动词名词化。我们已经看到了 look forward to/ contribute to/ dedicate to 后接 doing 的句子，下面我们来看看 look forward to/ contribute to/ dedicate to 后接名词的例句：

Each year I look forward to these events.

Team building activities contributed to a more positive team climate.

He had dedicated his life to a Palestinian state.

当然，除了动词加 to doing 的结构外，我们还能看到形容词加 to doing 或者名词加 to doing结构，他们的道理相通。让我们来看看下面这些例子。这里的 to 都是介词，我特别找来to sth. 和 to doing 的句子，让大家可以做一个比较，以便让大家更能认清这里 doing 的“名词本质”。特别值得注意的是，the key to doing 以及 the approach to doing ,在学术写作中出现的频率很高，而很多人并不知道这里的 to 后面需要加 doing。

Most football fans are far more addicted to watching football on TV than watching a game of football.

Do you think you're addicted to plastic surgery?

Students are encouraged to brainstorm together, as opposed to using a dictionary.

It emphasizes oral histories and sharing memories as opposed to material objects.

Letting kids choose their own books is the key to helping them read.

If passion is the key to happiness, this book is 450-plus pages of pure joy.

We used a thematic approach to data analysis.

Professional development is a common and necessary approach to improving teacher quality.