Can you please help me understand the difference between 'prepositional phrase' and 'adjective phrase'?
- He drives the car at high speed. (Prepositional phrase)
- The boy in the shop is my friend. (Adjective phrase)
My question is why these two phrases are different when both of them start with the preposition 'in'?
Best Answer
As BillJ said, it's essential that you know the difference between syntactic categories and grammatical functions.
A syntactic category is merely the class of the expression. At high speed and in the shop are both prepositional phrases because they are headed by prepositions, which are at and in, respectively. They are clearly not adjective phrases because they are not headed by adjectives.*
A grammatical function, on the other hand, is a relational concept.† It expresses the relation between the syntactic category and the whole clause. A function is said to be realized by a word or phrase. In the first sentence, the prepositional phrase realizes the function of a modifier that modifies the verb drive.
In the second sentence, in the shop is a prepositional phrase realizing the same function. However, it's not modifying a verb; it's modifying the head noun boy in the noun phrase the boy in the shop.
* Though, adjectives can head a noun phrase in a fused-head construction as in 'The poor'.
† Huddleston, Rodney and Geoffrey K. Pullum. A Student's Introduction to English Grammar, Cambridge University Press, 2005: 14.