Split infinitives have been a source of grammatical controversy for centuries. To understand split infinitives, you must first be able to recognize an infinitive, which you can think of as the plain form of a verb. Infinitives don’t reflect tense and don’t have to agree with any subject.
In English, infinitive verbs often begin with the word to, as in:
In the following sentence, the verb to proofread is an infinitive.
She likes to proofread every email and instant message she sends.
As you can see, to and proofread appear side by side. Unless there is a stylistic or content-related reason to separate the to from the rest of the verb, you should keep the two pieces of an infinitive together.
For example, the following sentence contains a typical example of unjustifiable infinitive splitting:
You need to completely fill out this form before starting the examination.
After all, you can write this instead:
You need to fill out the form completely before starting the examination.
In addition, avoid wide splits, as in the following example, where to is separated from evaluate by three words.
They want to carefully and accurately evaluate Sarah Brown’s credentials.
The following revision is much easier on the eyes and ears.
They want to evaluate Sarah Brown’s credentials carefully and accurately.
Many people mistakenly believe that regardless of the circumstances you should never, ever split an infinitive. A review of respected style guides, however, uncovers little or no support for this point of view, and in fact, the blanket veto on split infinitives appears to be grounded in grammatical myth.
The split-infinitive debate has its origins in Latin grammar, in which the split infinitive is an impossibility because the Latin infinitive is a single word. Since it wasn’t possible to split infinitives in Latin, some people argued, it shouldn’t be permitted in English. Over the centuries, their opponents have maintained that since English infinitives consist of two words, writers should take advantage of that flexibility. Indeed, award-winning writers such as the novelist Philip Roth split infinitives with both artistry and impunity.
You will perhaps recognize this famous split infinitive from television:
To boldly go where no man has gone before.
In this case, the adverb boldly divides to and go. But what are the alternatives? To go boldly where no man has gone before? Boldly to go where no man has gone before? The first alternative strips the phrase of its life; the second sounds silly. Hence, the Star Trek split infinitive is justified for stylistic and content-related reasons.