How do infinitives differ from prepositional phrases in their use in sentence?

Answer 1

Infinitives refer to a verb as a noun (or adjective/adverb): I like to eat. What do you like? (what is the direct object?) To eat. It is being used as a noun.

Prepositional phrases use "to" as any other preposition, as a part of a phrase which modifies another part of the sentence: I read to the girl. The phrase "to her" is modifying the verb in this case, read. How did you read? To her. An easier way to tell them apart is an infinitive will have a verb after "to". A prepositional phrase will need to have an object of the preposition, a noun. Therefore, it will have a noun after "to":

To eat.

verb, therefore an infinitive

To the girl.

noun, therefore a prepositional phrase