In the sentence: "I think the waiter forgot to charge me." The "to" is used as a preposition or to leave the verb in the infinitive, help me please. Yes, i am foreign xd

The 'to' is part of the infinitive verb form. It is not a preposition.

Explanation:

Remember this:

If a verb follows the word 'to,' then it is an infinitive. If the word that follows the word 'to' is an article, a pronoun, or a noun, then the 'to' is a preposition.

        prep.             infinitive

I went to  the store to buy  a car.            

second person

explanation:

note the heavy uses of "you", "your", as well as "he".

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