The infinitive form of the verb consists of the word "to" followed by the base form of the verb: "to be," "to serve," "to chop," etc. Inserting a word or words between the "to" and the verb of an infinitive creates what is known as a split infinitive. Prescriptive grammarians, who knew Latin grammar better than English, once decreed that a split infinitive was an error, but now it is growing increasingly acceptable even in formal writing. Nevertheless, some careful writers still prefer to avoid splitting infinitives altogether.

In general, you should avoid placing long, disruptive modifiers between the "to" and the verb of an infinitive. However, you must use your judgement when it comes to single-word modifiers. Sometimes a sentence becomes awkward if a single-word modifier is placed anywhere but between the elements of the infinitive:

[WRONG] The marketing team voted to, before they launched the new software, run an anticipatory ad campaign. (disruptive -- the infinitive should not be split)

[RIGHT] The marketing team voted to run an anticipatory ad campaign before they launched the new software.