So… is the to before an infinitive a preposition? If you have a sentence, e.g., “He decided to write a blog post on the topic,” is the to a preposition, or is it just a part of the infinitive?
It’s a tricky question, is the short answer. The detailed answer starts with the fact that our current to infinitive form is a survival not of the Old English infinitive form but of the “inflected infinitive” form (the infinitive inflected in the dative case, which was the normal case following the preposition to). This form indicated purpose, and took the preposition to (an example from Genesis: “Ðā ġeseah ðæt wīf ðæt ðæt trēow wæs gōd tō etenne” – “Then the woman saw that the tree was good to eat”). You will see, on the most excellent guide to Old English inflections in the world, that, for instance, witan (know) is, in inflected infinitive, tō witanne. Modern English equivalents of this usage include “he came to help” and “he prepared to depart.”
So why does that matter? Because in reality it’s still the case that the infinitive only takes to in certain circumstances. Now, it is a belief on the level of superstition that to is an inseparable part of the infinitive, and it is now completely standard to cite English infinitives with to. This is because with the loss of inflectional affixation, to is effectively the infinitive’s only unique distinguishing mark. And, indeed, the to is used more widely on infinitives now than it was in Old English – it is skipped mainly just after auxiliaries. But analytically it is still a separate piece.
There remains from this the issue of whether, coming before the infinitive, it is actually a preposition or not, and on that matter, allow me to quote from the Oxford English Dictionary:
in mod.Eng. the infinitive with to is the ordinary form, the simple infinitive surviving only in particular connexions, where it is very intimately connected with the preceding verb…. To a certain extent, therefore, i.e. when the infinitive is the subject or direct object, to has lost all its meaning, and become a mere ‘sign’ or prefix of the infinitive. But after an intrans. vb., or the passive voice, to is still the preposition. In appearance, there is no difference between the infinitive in ‘he proceeds to speak’ and ‘he chooses to speak’; but in the latter to speak is the equivalent of speaking or speech, and in the former of to speaking or to speech. In form, to speak, is the descendant of OE. tó specanne; in sense, it is partly the representative of this and largely of OE. specan.