[tspiteri]

From the Oxford A–Z of Grammar and Punctuation by John Seely:

split infinitive

The infinitive is the form of the verb made by adding to to its stem:

to go

Some traditionalists say that you should never place anything between the to and the stem. They argue that since the infinitive is a part of the verb it should never be split. So it is wrong to say to boldly go. You should instead say to go boldly or boldly to go.

There is no grammatical justification for this so-called 'rule', and people have been splitting infinitives for centuries. Indeed sometimes it is impossible to convey your meaning unless you do split an infinitive. For example:

Everyone else thought they were too young to really cope with adult responsibilities.

If you move really to another position you change the meaning of the sentence:

Everyone else thought they were too young really to cope with adult responsibilities.

Everyone else thought they were too young to cope really with adult responsibilities.

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[ColinWright]

In this example, I personally prefer "to cope fully" and thereby avoid the argument anyway.

But as I say elsewhere, I agree that sometimes splitting the infinitive is necessary to shade the meaning correctly.

[ahpeeyem]

"Everyone else thought they were really too young to cope with adult responsibilities"

If you word it like this, the emphasis of 'really' is moved to 'too young' but the sentence's meaning is the same, right?

[tspiteri]

But then 'really' modifies 'were too young', not 'to cope'.