 In a sentence such as John jumped straight into the pond we have a prepositional phrase that functions as an adverbial. So let's look at the internal structure of a prepositional phrase and start with the categorical analysis. Clearly straight is an adverb, into is a preposition and the pond is a noun phrase that consists of a determiner and a noun but here its suffices not to analyze the noun phrase any further but to present it as a triangle. Now clearly the preposition and the noun phrase form a constituent because we can replace into the pond by a proform for example by there as in John jumped straight there or by where when we build an echo question such as John jumped straight where. So together with the adverb which is the specifier of the prepositional phrase we get this structure where the internal prepositional phrase is represented as p bar and as usual is written with an apostrophe. So this is the structure of a prepositional phrase.