Prepositional phrases are often optional modifiers in the sentence rather than the central elements. Nonetheless, prepositional phrases appear over and over, and so it's worth examining how these phrases work in some detail.

We have already defined prepositions as a class of words that most commonly express relationships of space or time, or which mark syntactic functions.

Examples of Prepositions:

Spatial Relationship: behind the house
Temporal Relationship: after the party
Syntactic Function: the crux of the matter

Like other major word classes, prepositions are the heads of their own phrases. Prepositions are typically followed by a complement, called the object of the preposition. Most of the time, the object of the preposition is a noun phrase. In other words, the abstract phrase structure generally looks like this:

As we will see shortly, there are exceptions to this rule, but this pattern is so typical that it is worth memorizing. If you see a word that you think is a preposition, look for the noun phrase after it.