An infinitive is the form of a verb made by adding to to its stem (e.g. to go or to do).
It is often argued that there should never be anything between the to and the stem. In Latin, the infinitive is a single word. This has led to the belief that the infinitive in English should be treated as a single unified concept, and that it is bad grammar to separate the two parts.
It is not bad grammar to split an infinitive; there is no grammatical reason not to. It is sometimes impossible to convey your meaning without splitting the infinitive. Sentences quite often read better when the infinitive has been split.
The most famous example of a split infinitive is probably to boldly go (from Star Trek). Would to go boldly or boldly to go have the same emphasis? No. Are they more natural? No.
As with ending a sentence with a preposition, there are circumstances where it would be more appropriate to avoid splitting the infinitive. It might be, for instance, jarring to split it.
You may also find that some publications would rather that you didn’t split infinitives lest you upset people by doing so. For that reason, The Economist’s Style Guide sadly declares that the ban on split infinitives is ‘pointless’ but that you should observe it anyway.
I’m not sure how healthy it is to obey a ‘rule’ that isn’t a rule just to keep your readers from confronting that it isn’t a rule.
If you want to read people agree with me, here are some books you should take a look at:
- For Who the Bell Tolls: One Man’s Quest for Grammatical Perfection, David Marsh.
- Fowler’s Dictionary of Modern English Usage, Jeremy Butterfield (editor).*
- Oxford A–Z of Grammar and Punctuation (Second Edition), John Seely.
- Oxford Guide to Plain English (Fourth Edition), Martin Cutts.
- The Chicago Manual of Style (Sixteenth Edition).
- The Penguin Writer’s Manual, Martin Manser and Stephen Curtis.
* Fowler’s suggests that you should avoid splitting infinitives but also states that it is acceptable and often necessary.
(To make things even more complicated, I recommend Trask’s The Penguin Dictionary of English Grammar for an explanation of why you aren’t actually splitting an infinitive at all.)
Thank you to Lucy aka Blondeusk for suggesting this post.