举例说明一下：

You have a nice chair in which you can sit.

You have a nice chair in which to sit.

You have a nice chair to sit in.

You have 3 minutes in which you can answer the question.

You have 3 minutes in which to answer the quesiton.

You have 3 minutes to answer the quesiton.

以上三种结构都是定语。用从句显得有些臃肿，用紧缩定语从句是很正式的说法，只用于正式书面语。直接用不定式是日常非正式用法。非正式不是不正确，而是非正式场合的日常交际用法。是标准的英语表达。就像我们用你和您要分场合一样。

He had three years in which to establish himself as Prime Minister.

=He had three years when he would establish himself as Prime Minister.

=He had three years when to establish himself as Prime Minister.

以上句子结构实际上市属于定语从句简化后的形式，简化后的介词“in”必须加上，不可以省略，例如下面：

这个穷人没有房子住。

The poor man has no house in which he can live.

=The poor man has no house to live in.

=The poor man has no house in which to live.

(备注：以上介词in+关系代词which用来作不定式地点状语）

这个乞丐没有钱购买食物。

The beggar has no money that he can buy food with.

=The beggar has no money to buy  food with.

=The beggar has no money with which to buy.

(备注：以上介词in+关系代词which用来作不定式方式状语）