The accusative case is used for the direct object (that's the noun or pronoun to which an action is done). It's like the shark in Peter ate the shark where shark is the noun that gets eaten. It also follows certain preopstions (words like for, through and without).
Only singular (i.e. not plural) masculine nouns change in the accusative. All the determiners and adjectives that stand before these always end in -en. The other nouns stay the same as in the nominative.
Some pronouns also change. The most important ones to remember are ich (I) becomes mich (me) and du (you) becomes dich (you). As you can see, that means that not all pronouns change in English to mark the object either.
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