Infinitives include: to become, to enter, to draw, etc. Prepositional phrases include: to him, to this address, to the mountains, to my house, etc. An infinitive phrase uses an infinitive plus a modifier, object, complement or actor. There can be more than one of these and they may not all be present in the phrase.

Why isn't an infinitive a prepositional phrase?

A prepositional phrase expresses time and location and can sometimes begin with “to.” Even though they also use the word “to,” prepositional phrases are different then infinitives because infinitives combine “to” and a verb, while prepositional phrases combine “to” with a noun or pronoun.

Is the infinitive to a preposition?

If you are reading some English, you can usually see the answer to this question. If to is followed immediately by a simple verb, it is part of an infinitive. If to is followed by a noun construction, it is a preposition. That's the easy and recognizable part.