Prepositions are always used with a noun or pronoun in a prepositional phrase, for example in the sea, to the cinema, behind the clouds, after breakfast, with her.
Notice how in English, we say: This is for him (not he); He went with William and me’ and Give the sweets to Helen and me (not I). These examples show how in English, prepositions (italicised here) really ‘take’ (are followed by) the objective, or accusative, case. The accusative case is something we'll come back to later.
In Latin, too, some prepositions govern the objective/accusative case; others govern another case (the ablative). This is something we have to learn, along with the preposition itself, as a vocabulary item.
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