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.