Prepositions in Latin function similarly to the way they do in English; they link a noun (or pronoun) with another word. The problem with Latin prepositions for English speakers is that Latin prepositions are paired with either the accusative or the ablative case. Knowing which to use when translating English to Latin is a matter of memorization. Translating from Latin to English is a much simpler situation. Consider the following examples:
Location: Caesar in urbe superatus est.
Caesar was defeated in the city.
Direction: Miles ad Caesarem Ambulavit.
The soldier walked toward (or to) Caesar.
Notice that, like English, the preposition precedes the remainder of the prepositional phrase. This makes them easy to recognize; whenever a preposition is encountered in Latin, look for a subsequent noun in either the accusative or ablative case and see if a prepositional phrase is employed.
One important note worth mentioning is with the use of the preposition “to." Recall that words functioning as indirect objects in a sentence are often identified in English with the words “to" or “for" preceding them. Latin uses the dative case to identify an indirect object and translating them to English requires the addition of the telltale words “to" or “for." Consider the following Latin sentences:
Puer puellae rosam dat.
The boy is giving a rose to the girl.
Puer ad puellam ambulat.
The boy is walking toward (or to) the girl.
Notice that the phrase “to the girl" appears in both sentences. However, in the first sentence, “puellae" is an indirect object in the dative case. This phrase answers to whom the rose (the direct object) is given.
In the second sentence, “ad puellam" is a prepositional phrase indicating to whom (or toward whom) the boy is walking. It is important to distinguish these two very different constructions. Latin prepositional phrases using a preposition and a noun in either the accusative or the ablative case do not replace use of the dative to indicate indirect objects. This is an important lesson that must be memorized for translation from English to Latin.