Grammar: The Split Infinitive

 

A verb is described as infinitive when it is not located in time (when it is not tensed). The bare infinitive is the form in which the verb appears in
dictionaries
 (play, clean, think, argue), but the form in question is called the full-infinitive or the to-infinitive 

      to play
      to clean
      to think
      to argue

Convention declares that this form of the infinitive should never be ‘split’. It should, that is, be treated as a sealed unit with no words intervening.
If we want to modify the verb (describe
how to play, how to clean and so forth), then the adverbs must come before or after the infinitive, not 
within 
     

      happily to play (correct)
      to play happily (correct)
      to happily play (split infinitive)

      thoroughly to clean (correct)
      to clean thoroughly (correct)
      to thoroughly clean (split infinitive)

      carefully to think (correct)
      to think carefully (correct)
      to carefully think (split infinitive)

      strongly to argue (correct)
      to argue strongly (correct)
      to strongly argue (split infinitive)

The rule does not apply where the intervening ‘word’ is actually a component of the verb. In such cases, a hyphen is normally used      

      to remotely-control (correct)
      to self-certify (correct) 
      to word-process 
(correct)

But the rule is also without foundation. Most sources agree that it is simply a caprice of nineteenth-century grammarians who felt that English
should emulate Latin, where the infinitive comprises a complete and indivisible word. It is also very much a British preoccupation; Americans have
few qualms about splitting their infinitives. Indeed, the much-quoted ‘blunder’ comes from the American television series Star Trek where the
Starship Enterprise is said to boldly go.

From the point of view of clarity, moreover, it is difficult to find anything wrong with the split infinitive. On the contrary, ambiguity is more likely to
occur by a rigid application of the rule 

      The guard asked the passenger to kindly extinguish his cigarette.

Any attempt to ‘correct’ this sentence results in a loss of clarity       

      The guard asked the passenger kindly to extinguish his cigarette.
      (Kindly seems to be modifying the guard’s request rather than the passenger’s
      compliance.)

      The guard asked the passenger to extinguish kindly his cigarette.
      (The word order here sounds distinctly foreign.)

      The guard asked the passenger to extinguish his cigarette kindly.
      (Now the guard appears to be asking the passenger to dispose of his cigarette
      humanely.)

The rule against splitting the infinitive has considerably more to do with social propriety than it has with grammar, rather like Estella’s disgust with
Pip in Great Expectations: He calls the knaves, Jacks, this boy. To adapt a remark by the lexicographer H. W. Fowler,1 we might say that there
are
 three classes of people in respect of the split infinitive: those who know of the rule and know it to be groundless; those who know of the rule and
take it seriously; and those who would not know a split infinitive if they tripped over one. The problem is that those in the first group do not want to be
mistaken for those in the third by those in the second.

Whether we like it or not, then, the rule has come to be regarded as an indication of one’s education, so it is probably sensible to follow it wherever
practical but to abandon it where common sense directs.

 

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 1 The English-speaking world may be divided into (1) those who neither know nor care what a split infinitive is; (2) those who do not know, but care
   very much; 
(3) those who know and condemn; (4) those who know and approve; and (5) those who know and distinguish... Those who neither know
   nor care are the vast 
majority, and are happy folk, to be envied by the minority classes. (H. W. Fowler, A Dictionary of Modern English Usage, OUP,
   1926)