What is the phrasal category of the bold constituent in the following sentences: The house across the street is haunted. The girls and their...

In the first sentence, the phrase begins with the preposition across:
The house across the street.

A prepositional phrase begins with a preposition and usually ends with a noun or a pronoun. A noun is a person, place, or thing; a pronoun is a word that replaces or refers to a noun, words such as he, she, it, etc.

A prepositional phrase will also contain modifiers such as articles (a, an, the). Professor of English and Humanities Robin L. Simons, author of the Grammar Bytes website, gives us the following examples of prepositional phrases:

• on the bathroom floor


• in the vegetable bin


• from Beverly

A prepositional phrase will also act as either an adjective or an adverb. Adjectives describe nouns; adverbs describe either verbs (action words), adjectives, or other adverbs. Professor Simons gives us the following sentence example containing a prepositional phrase acting as an adjective:
The sweet potatoes in the vegetable bin are green with mold.
Here, the phrase in the vegetable bin is further describing the compound noun sweet potatoes by answering the question, what sweet potatoes? Therefore, we know the phrase is acting as an adjective.

The following is my own example of a prepositional phrase acting as an adverb:

• While in the library, she spoke in a quiet whisper.

Here, the phrase in a quiet whisper is further describing the verb spoke by answering the question, how? Therefore, we know the phrase is acting as an adverb.

The phrase in question in the first sentence begins with the preposition across and ends with the noun street. The entire phrase is also further describing the noun house--it's telling us more about the house by answering the question, what house? Therefore, we know the phrase is a prepositional phrase.

In the phrase of the second sentence, the nouns girls and mother are joined by the conjunction and:

• 
  The girls and their mother went swimming.

Noun phrases are phrases that contain nouns and their modifiers. A noun is a person, place, or thing; modifiers that can be found in a noun phrase include articles (a, an, the), possessive nouns (Uncle Jim's dog), adjectives (the cute dog), and participles (the mewing kitten). Professor Simons gives us the following examples of noun phrases:

• the dog


• Aunt Audrey's dog


• the barking dog

A compound subject occurs when we join two or more noun phrases with a conjunction. The compound subject creates one long noun phrase. The editors of the Purdue University's Online Writing Lab give us the following examples of sentences containing noun phrases made up of compound subjects:

• 
  She and her friends are at the fair.


• 
  The book or the pen is in the drawer.


• 
  The boy or his friends run every day.

Therefore, we know the phrase in the second sentence is a noun phrase with a compound subject.