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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'?

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  • I think you are confusing word category (part of speech) with function. A preposition phrase (PP) has a preposition as its head, and an adjective phrase (AdjP) has an adjective as head. In your first example "at high speed" is a PP with "at" as head functioning as a modifier in clause structure. In the second, "in the shop" is also a PP, with "in" as head, and its functions is that of modifier of "boy".
    – BillJ
    Sep 18 '17 at 11:20
  • Please don't ask unrelated questions in the same post. If you have two questions, open two questions.
    – aschepler
    Sep 18 '17 at 11:37
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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.

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  • Prepositional phrases don't modify verbs at all.
    – user61367
    Sep 18 '17 at 13:43
  • @RolaAbu-Ghazaleh Hmm.. I wonder why not. In the OP, the PP clearly describes how he drives. So that means it's an adjunct of manner, which is a modifier.
    – user178049
    Sep 18 '17 at 13:51
  • Adverbs modify verbs, adjectives modify nouns, prepositional phrase adds extra information to the clause as circumstances He drives the car or He drives the car at high speed. If you wanna modify the verb, you can say * He often/usually/ drives the car at high speed* for example.
    – user61367
    Sep 18 '17 at 14:00
  • @RolaAbu-Ghazaleh PPs don't necessarily add circumstantial information. I laugh at him. at him is a PP but it heads the noun that's the 'patient' of the action.
    – user178049
    Sep 18 '17 at 14:10
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    @RolaAbu-Ghazaleh The OP's question has been accurately answered by user 178049, and by me in my original comment to the OP. To repeat: "at high speed" is a PP in clause structure -- it modifies the VP "drives the car", and thus is semantically associated very closely with the verb. The PP is an adjunct of manner -- it tells us how he drives the car.
    – BillJ
    Sep 18 '17 at 14:49
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  • The prepositional phrase ,which contains a preposition together with it's associated noun phrase, can either qualify the head word in a noun phrase or act as a circumstance in a clause**

    • He drives the car at high speed. The preposition phrase at high speed acts as a circumstance of the clause. The boy in the shop is my friend. The preposition phrase here in the shop modifies the noun the boy by telling us which boy in particular is her/his friend. Like in The image I remember is of a man with a rifle {qualifier} / The accused is supposed to have shot the victim with a rifle { circumstance}. There's no an adjective phrase in your examples.
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  • I know but here in the shop is a qualifier for the noun phrase The boy
    – user61367
    Sep 18 '17 at 13:40

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