Here's a simple way to remember it:
* Preposition: A word that shows the relationship between a noun or pronoun and another word in the sentence. (Examples: on, in, at, to, from, with, by, for, etc.)
* Prepositional Phrase: A group of words that includes a preposition and its object (the noun or pronoun that the preposition is referring to).
For example:
* "On the table" is a prepositional phrase.
* "On" is the preposition.
* "The table" is the object of the preposition.
So, a prepositional phrase must start with a preposition to function correctly.