Prepositions- are words placed before nouns or pronouns to form a phrase modifying another word in the sentence, indicating spatial position or logical relationship. Prepositions are never followed by a verb (unless it is a gerund, which in this case is really a noun in verb form.) 

Example: Locating the Preposition

The road to hell is paved with good intentions

Put the food on the table.

She lives in Florida.

Your science book is under your math book. 

Infinitives- If a preposition is followed by a verb (not including gerunds) it is an infinitive. 

Example: Locating the Infinitive

I would like to swing outside now.

She used to smoke a lot. 

Example: Preposition

The French aren’t nice to Americans. (to is the preposition followed by the noun Americans)

She isn’t used to working. (to is the preposition modifying the noun-gerund working)