What does a prepositional phrase do in a sentence?

A prepositional phrase normally acts as an adjective or an adverb.

As an adjective, the phrase modifies a noun or a nominal and comes immediately after the word it modifies:

We bought the house on the corner.

[on the corner modifies the noun house]

One of the baby birds has fallen out of the nest.

[of the baby birds modifies the pronoun one]

I could hear only the sighing of the wind.

[of the wind modifies the gerund sighing]

As an adverb, the phrase usually modifies a verb. The phrase may be next to the verb or at the beginning or end of the sentence:

The letter sat on the desk all week.

[on the desk modifies the verb sat]

The panther crept silently along the narrow ledge.

[along the narrow ledge modifies the verb crept]

With a puzzled look, Jason followed his sister.

[With a puzzled look modifies the verb followed]

An adverb prepositional phrase may also modify a verbal (i.e., a present or past participle, a gerund or an infinitive):

Racing toward the finish line, Georg left the other runners behind.

[toward the finish line modifies the present participle racing]

The passengers seated at the back couldn’t see.

[at the back modifies the past participle seated]

I don’t mind sailing in rough weather.

[in rough weather modifies the gerund sailing]

Do you want to go to a restaurant for lunch?

[to a restaurant and for lunch modify the infinitive to go]

Some adverb prepositional phrases modify adjectives:

The children were afraid of the barking dog.

[of the barking dog modifies the adjective afraid]

Sometimes an adverb prepositional phrase may follow the linking verb be:

Samina was in the garden.

Occasionally, a prepositional phrase may act as a noun, but this type of structure is unusual in formal writing:

After midnight is the best time to view the comet.

[After midnight acts as the subject of the verb is]