以下解释摘自《现代英汉综合大辞典》

the为定冠词：

[用于形容词、副词最高级前, 有时也用于形容词比较级前]

the most interesting book I have ever read

我所读过的最有趣的书

the elder of the two brothers

两兄弟中年纪大的一个

The workers and peasants suffered the most deeply in the old society.

工人和农民在旧社会受苦最深。

the为副词：

1.[加在形容词, 副词的比较级前]表示“更, 越发, 反而”

That will make it all the worse.

那将 使事情愈搞愈坏。

He is none the wiser for all his experience.

尽管他有这么多的经验, 他还是没明白。

If you start now, you will be back the sooner.

假使你现在动身, 就能回来得更早一些。

2.[用于The more ..., the more 结构中, 前者作关系副词, 后者作指示副词]表示“愈...愈...，越...越...”

The sooner the better.

愈快愈好。

The higher the tree, the stronger the wind.

树越高, 风越大。

3.[用于副词最高级前]

Among them, he studies the hardest.