Question

When does 'to' function as a preposition?

Answer 1

A preposition is a phrase or word that introduces another part of speech. Notice that it is called "pre-position" since it comes before whatever phrase it is introducing :)
A few examples of prepositions are: with, without, about, by through, under, below and yes, TO is a preposition as well.
Prepositions can function as nouns, adjectives or adverbs.

Answer 2

Also, prepositions can convey a few relationships such as:
1. Comparison (like, as...)
2. Direction (toward, TO, through...)
3. Place (at, by, on...)
And a few more ^^
An example of a prepositional phrase with 'to' in it is this:
'You make me sad! I have never been TO Australia!'

So basically 'to' can function as a preposition when you are talking about direction (e.g. I'm going TO France tomorrow).
I hope my ranting helped answer your question :3

Answer 3

Easy way to remember prepositions: anything a frog can do to a log (the from can go THROUGH it, AROUND it, OVER it, TO it, etc.)