A preposition is a word which accompanies a noun (or a pronoun) to provide information such as the location of the action of a sentence in space or time, e.g. in, towards, against, with, among, from, after, about.
Prepositions are always used with a noun or pronoun in what is called a ‘prepositional phrase’, for example: towards the woman, near the cinema, behind the clouds, after breakfast, with her.
Notice how English says: ‘This is for him’ (not ‘for he’); ‘He went with me ’ (not ‘with I’) and ‘Give the sweets to us ’ (not ‘to we’). That is to say, English tends to use a particular form of a pronoun when it is used in a prepositional phrase. In Latin, it is not just the forms of pronouns that change when they are used with prepositions: the forms of nouns change, too. The Latin for ‘slave’, for example, is servus ; but ‘with the slave’ in Latin is cum servo (i.e. the final – us of servus has changed to – o).