Here you'll get all the information you need to internalise the accusative.
Accusative an objective case and the direct object.
But what does it really mean?
The accusative of a noun is the grammatical case used to indicate the direct object of a transitive verb. In English, the only words that occur in the accusative are pronouns: "me", "him", "her", "us" and "them". The spelling of these words changes depending on how they are used in a sentence.
In German, pronouns and also articles are changed: the masculine articles der and ein become "den" and "einen" in the accusative. The feminine, neutral and plural articles do not change. The masculine pronoun "er" (he) becomes "ihn" (him), as in English.
The direct object (accusative) is the recipient of the action of a transitive verb. When one buys or has something, the "something" is the direct object. The subject (the person who buys or has something) acts on this object.
What is a transitive verb?
You can find out if it is a transitive verb by saying it without an object. If it sounds strange or wrong/incomplete and seems to need an object to sound right, it is probably a transitive verb, for example: I have. He buys. Both sentences answer the implicit question "What?". What have you? What did he buy? And whatever that is, is the direct object and should be in the accusative in German.
With an intransitive verb like "to sleep", "to die" or "to wait", on the other hand, no direct object is required. You cannot "sleep", "die" or "wait" something. To be and to become are not exceptions, as they are intransitive verbs that act like an equal sign and cannot take an object.
Important: Some verbs in English and German can be both transitive and intransitive, but the key is to remember that in German you have the accusative when you have a direct object.
The Germanic word for the accusative, wenfall, reflects the change from der to den. The interrogative word in the accusative case is wen (whom).