The words in prepositional phrases can answer a number of different questions about the words they modify.
What kind of? Which? What? Whose?
Many prepositional phrases help to describe or identify the word that they modify:
The seeds of apples are small. [What kinds of seeds are small?]Where? How Far?
Some prepositional phrases indicate where things are or where they happen.
The tree in the back yard was hit by lightning."On the couch" explains where she took a nap. Some phrases can answer more than one question. "In the back yard" tells both which tree and where it is. "In the living room" explains both which couch, and where it is.
When? How long?
They left after dinner.Why?
Because of the rain, they went inside.How?
The Johnsons went to Hawaii by boat.Under What Conditions?
The twins walked home in the rain.