Infinitive "To" v.s. Preposition "To"

My answer to yesterday's question was/is:

If you change the words after "to" to the pronoun "it" and the sentence makes sense, then the "to" is a preposition and thus should be followed by a gerund (V-ing). Otherwise, it is part of an infinitive and a root verb should come after it.

e.g.

1. I'm looking forward to ______ (go) home next month.

2. I'm looking forward to it.

The second sentence makes sense and is grammatically correct, so we use the gerund "going" after "to."

3. I'm looking forward to going home next month.

4. I want to ____ (go) home next month.

*5. I want to it.

The fifth sentence doesn't make sense, so the "to" is not a preposition but part of an infinitive and "go" is the right form to fill in.

6. I want to go home next month.

Why? Because after a preposition is always a noun or a pronoun. Gerund is the noun form of a verb.

No one has ever taught me the rule. When I was studying at graduate school in the States, I asked a grammar professor about this confusing "to," and he suggested I learn what to follow "to" along with a phrase. For example, when learning "be dedicated to," I should remember it is followed by a gerund.

I came up with the rule just by applying a general basic pattern in English grammar: Preposition + Noun/Pronoun