A maths teacher asked me recently whether the rules regarding split infinitives have become less strict. Undoubtedly, they have.
To go back, a split infinitive happens when there's an adverb between ‘to’ and a verb. For example, ‘She used to secretly admire him’ and ‘You have to really watch him’. Whereas in general, rules have weakened and people have become more accepting of split infinitives, some still say they are grammatically incorrect and should be avoided. These people are likely to rewrite the above examples as ‘She used secretly to admire him’ and ‘You really have to watch him’.
There is no real justification for their objection, which is based on comparisons with the structure of Latin. People have been splitting infinitives for centuries in order to avoid clumsy sounding alternatives, as seen in the example above.
Split infinitives also change the meaning of a sentence. ‘You have to really watch him’ (it’s important he’s watched) is different to ‘You really have to watch him’ (you have to watch him very closely).