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Adverbs that tell us how an activity is done can be positioned before the verb for emphasis.

Adverbs that tell us how or how often can be placed between the auxiliary and main verb.  "split"

Most adverbs are positioned after the verb.  (If the adverb modifies another adverb or adjective, it is placed before it.)

The adverb is placed between the auxiliary verb and the main verb — the auxiliary and main verb are split apart.

The adverb is placed after the auxiliary and main verb  — the auxiliary and main verb are together.

 An adverb can be placed before the sentence as a modifier to the whole clause.

Commonly, an adverb is placed between the auxiliary verb and the main verb.  ("split")

 An adverb can be placed after the verb for less emphasis or to avoid a "split verb".

Any speaker who has not been brainwashed by the split-verb myth can sense that these corrections go against the rhythm and logic of English phrasing. The myth originated centuries ago in a thick-witted analogy to Latin, in which it is impossible to split an infinitive because it consists of a single word, like dicere, “to say.” But in English, infinitives like “to go” and future-tense forms like “will go” are two words, not one, and there is not the slightest reason to interdict adverbs from the position between them. — Steven Pinker "Oaf of Office"

"No rational basis for this prescriptivist rule. "
Prescriptive condemnation of the 'split infinitive' did not rise until the second half of the nineteenth century.  The construction can be found in the literature of the preceding several hundred years, but it became more popular in English writing as the nineteenth century went on, and the adoption of the rule in prescriptive grammar reflected disapproval of this change.  No reason was ever given as to why the construction was supposedly objectionable, however. 
 

First, all the evidence points towards the reality of the feeling that it is "wrong" to split infinitives….On the other hand it is clear that rigid adherence to a policy of non-splitting can sometimes lead to unnaturalness or ambiguity…. Preference.  No absolute taboo should be placed on the use of simple adverbs between the particle to and the verbal part of the infinitive.  'Avoid splitting infinitives whenever possible, but do not suffer undue remorse if a split infinitive is unavoidable for the natural and unambiguous completion of a sentence already begun.' — Burchfield. The Spoken Word (1981) 
 

It should be noted that the term 'split infinitive' is a misnomer: nothing is being split.  In Latin there is an infinitive form of the verb, which is traditionally translated into English by means of to + the plain form.  Latin amare, for example, is translated to love. But where amare is a single word, to love is not: it is a sequence of two words.  Thus the fact that no adjunct can be positioned within amare provides no basis for expecting it should be contrary to grammatical principles to position one between to and love.   — Huddleston & Pullum (CaGEL 581)

Split Infinitives. A. Generally. Although few amrmchair grammarians seem to know it, some split infinitives are regarded as perfectly proper…[Garner cites several grammarians.] B. Splits to Be Avoided. If a split is easily fixed by putting the adverb at the end of the phrase and the meaning remains the same, then avoiding the split is the best course:
Split: "It is not necessary to here enlarge upon those points."
Unsplit: " It is not necessary to enlarge upon those points here." 
Such capriciously split infinites only jar the reader.  — Garner (767)
Stage 5: Fully accepted on Garner's Language Change Index.