A preposition forms a phrase with a noun or pronoun, called the preposition’s object. The preposition links the object to another word or element in the sentence. A prepositional phrase usually functions as an adjective or adverb, modifying a noun, pronoun, verb, adjective, or adverb.
In most cases a prepositional phrase functions as a unit.
The cat hid under the bed.
Above, the preposition under forms the prepositional phrase under the bed.
Please be home before midnight.
In the sentence above, the preposition before forms the prepositional phrase before midnight.
The child called out to her.
Above, the preposition to forms the prepositional phrase to her.
Despite what you may have heard, it’s perfectly acceptable to end a clause or sentence with a preposition. Keeping prepositions at the end of clauses usually sounds more natural and less formal because it follows spoken English.
What was the report about? (About what was the report? sounds formal and unnatural.)
The rescue group we adopted our dog from needs volunteers. (The rescue group from whom we adopted our dog needs volunteers is grammatically correct and acceptable but sounds more formal than the original sentence.)
That’s not the organization I wrote to. (That’s not the organization to which I wrote is grammatically correct but sounds more formal than the original sentence.)