In English the infinitive is of the form “to be”, “to go”, “to sing” etc.
To split an infinitive is to interpose a word (generally an adverb) as in:
“to boldly go”
Now although I personally would put ‘boldly’ after the infinitive here (I do not watch Star Trek), this is just a matter of style or taste. However, there is a fetish, especially in British English, that regards splitting an infinitive as the most serious grammatical crime that one can commit. This is, in fact, nonsense, as the highly respected author Fowler wrote in his Modern English Usage. I quote at length from an extract from a recent edition.
To avoid an unfounded charge of illiteracy, you might feel the need to
avoid splitting your infinitives, but don’t feel obliged to. If there
is no way to avoid the split without introducing ambiguity, go ahead
and split without regret, and refer your uninformed critics to Fowler.
He called slavish obedience to such unfounded rules a fetish.
A Daily Telegraph report (27th August 2003) said:
A family doctor who installed a camera secretly to film a woman using
his bathroom ...
which is unclear: was the installation secret or is the sentence a
clumsy attempt to avoid a split infinitive?
Some other sentences where splitting is unavoidable:
I have to refuse to promise to help to write it.
If you want to refuse
to definitely promise, how could you do it without splitting and
As soon as I read the book I decided to immediately agree to film it
if she asked me.
In this case immediately splits to agree, but where
else could it sensibly be put?
Sometimes people avoid splitting a non-infinitive: to be greatly
regretted is not a split infinitive (the verb is “to be”), but people
feel the need to write greatly to be regretted.
Why is splitting an infinitive worse than splitting any other tense?
Why do people who would never dream of splitting an infinitive happily
split the future tense, as in I will probably be going?