A postposition is a type of word that is used to indicate some form of spatial or time relationship between two objects and occurs after the word or phrase it complements. For example, in the sentence "I went to the store two days ago," the word "ago" is a postposition that is used to indicate the phrase "two days" as the timeframe for the action in the sentence. These types of words are fairly rare in English, in which prepositions such as "to" or "on" are more frequently used, though postpositions are more common in some other languages. Both of these kinds of words are more generally referred to as adpositions.
The basic function of a postposition is to provide information that relates one word or phrase to another, often in terms of space or time. This functionality is much the same as prepositions within English, though the differences between them is indicated by the prefix in each term. The "pre-" in preposition indicates that these words come before the complement that they are providing information about, while "post-" in postposition refers to its placement after the word or phrase. Each type of adposition is used with some form of subject or object in a sentence, and a word or phrase that is used to provide more information about it.
In English, the word "ago" is the most common postposition and provides information about when an action was performed. It is used in the sentence "She was born 12 years ago," in which "She" is the subject of the sentence and "was born" is the verb phrase that indicates the action of the sentence. The postposition "ago" occurs after the phrase that it is a complement to, "12 years," and it indicates that this amount of time in the past is when the action in the sentence occurred. Other languages such as Chinese, Korean, and Finnish include more postpositions than English.
Some words can function as either a preposition or a postposition, depending on how they are used within a sentence, and these are often called ambipositions. The word "through," for example, can typically be used as a preposition such as "We burned firewood to keep warm through the whole winter;" it comes before the phrase "the whole winter" to indicate the time during which the action of the sentence occurred. It can also be used as a postposition, however, and the sentence can be rewritten as "We burned firewood to keep warm the whole winter through." This is a somewhat archaic wording, and may seem strange or unnecessarily formal, but it is technically accurate and provides the same function within the sentence.