不定式动词(Infinitive)的形式是：“to + 原动词”, 如：to come, to go, to play, to read, to listen, to sing等等。

在一般情况下，“to + 原动词”必须靠拢，但在某些情况里，“to”和原动词可以被适当的副词(Adverb)分开，在某些动词句型里，“to”则必须省略。

在什么情况下，不定式动词的两部份可以分开呢?分开的目的何在?在大多数场合里，不定式动词两部分不分开，只有在特殊环境里才准许让副词加入，以便更准确地，更自然地反映某些实际情况。看看下面三个句子：

(1) Bob failed completely to fulfil his duties.

(2) Bob failed to fulfil his duties completely.

(3) Bob failed to completely fulfil his duties.

在(1)里，状态副词“Completely”既可修饰谓语动词“failed”，也可修饰不定式动词“to fulfil”。在 (2) 里，“completely”和被修饰的不定式动词离开太远，力道不足，效果欠佳。在 (3) 里，“completely”夹在不定式动词两部份之间，把它修饰得既自然又贴切;这个位置，在三个句子中，可说是最恰当的了。

同样的，(4)比(5)紧凑;(6)比(7)扎实：

(4) It was wrong for Mr Lim to suddenly quit the job.

(5) It was wrong for Mr Lim suddenly to quit the job.

(6) Is it possible for a chaotic country to further develop its economy?

(7) Is it possible for a chaotic country to develop its economy further?

由于不定式动词的两部分在适当时分开有其优点，这种用法，在现代英语里越来越普遍。例如：