What is a split infinitive? And when should you avoid using one?

Infinitives are two-word forms of verbs, such as to run, to research, or to suggest. A split infinitive happens when you insert an adverb in between to and its verb in an infinitive phrase. Consider the following:

to quickly run

to diligently research

to calmly suggest

The traditional grammatical rule is to avoid this split, putting the adverb after the infinitive:

to run quickly

to research diligently

to suggest calmly

This way, you know what the action is (run) before you know the manner in which it is happening (quickly).

But there are always exceptions to the rule. Sometimes a split infinitive works better than an infinitive. The most famous example is:

to boldly go where no man has gone before

If Star Trek hadn't split the infinitive, and had just used "to go boldly where no man has gone before," it wouldn't be the catchphrase it is today.