Prepositional phrases

The bolded phrases are examples of prepositional phrases in English:

She is on the computer.
He could hear her across the room.
Sarah walked down the ramp.
They walked to their school.
Garrett ate in the kitchen.
Prepositional phrases have a preposition as the head of the phrase.

The first example could be diagrammed (using simplified modern notation):

IP
/ \
NP VP
| | \
N V \
| | PP
She is / \
/ \
P NP
| / \
on Det N
| |
the computer
Where by convention:

IP = Inflectional phrase (sentence)
NP = Noun phrase
N = Noun
VP = Verb phrase
PP = Prepositional phrase
P = Preposition
Det = Determiner
The diagram shows that the prepositional phrase in this sentence is composed of two parts: a preposition and a noun phrase. The preposition is in the head position, and the noun phrase is in the complement position. Because English is a head-first language, we usually see the head before the complement (or any adjuncts) when we actually read the sentence. (However, the head comes after the specifier, such as the determiner "the" in the noun phrase above.)

See adposition for more examples of complements found in prepositional phrases.

Prepositional phrases generally act as complements and adjuncts of noun phrases and verb phrases. For example:

The man from China was enjoying his noodles. (Adjunct of a noun phrase)
She ran under him. (Adjunct of a verb phrase)
He gave money to the cause. (Oblique complement of a verb phrase)
A student of physics. (Complement of a noun phrase)
She argued with him. (Complement of a verb phrase)
A prepositional phrase should not be confused with the sequence formed by the particle and the direct object of a phrasal verb, as in turn on the light. This sequence is structurally distinct from a prepositional phrase. In this case, "on" and "the light" do not form a unit; they combine independently with the verb "turn".

Another common point of confusion is that the word "to" may appear either as a preposition or as a verbal particle in infinitive verb phrases, such as "to run for president".