Infinitives - anorak note:
The infinitive form of a verb, such as "swim", is "to swim".
It is argued by some that this phrase, which is just one particular form of the verb, is effectively all one word and should not be split, any more than one would write "sw [insert some extra words here] imming", and that when you split an infinitive the isolated word "to" has no grammatical legitimacy on its own.
There is also a view that to split an infinitive in thie way also leads to rather clumsy English.
However there is not a "rule" about this; it is much more a matter of preference, good style and preferred usage. Those who claim that there is a rigid rule about it and that spliting an infinitive is always definitely "wrong" are actually betraying their own ignorance - challenge them to find an authoritative source for their assertion! As they say, a little knowledge is a dangerous thing.
Perhaps a good guideline is to avoid it and only to split an infinitive when the alternative would sound even more clumsy or if there is some other good reason for doing so.
The classic example, as quoted above, is "to boldly go". Would "To go boldly" or "boldly to go" have the same ring to them? Of course not. Good English is frequently a matter of good style.
(As a further arrogant anorak suggestion: when chooing a word, think about what the word actually means - look it up if neccessary - and consider possible alternatives. "Disintersted" does not mean "un-interested"; "discrete" is a different word from "discreet" and has a quite different meaning; likewise "infer" and "imply" have different meanings and usages. "Fantastic" is ludicrously overworked and of course literally just means "like a fantasy" - in other words, the thing being described does not really exist at all!)