The accusative case is used to signal the direct object (the thing that is being acted upon). We also use the accusative case after certain verbs and prepositions.
The questions for the accusative case are "Wen?" (Who?/Whom?) or "Was?" (What?)
What does the man have? – ein Pferd!
The accusative case is always used after the verb "haben." That’s because haben always need a direct object.
What does the boy give to his friend? – Die Blumen!
"Die Blumen" is the direct object, which is being acted upon. They are what is being given.
In the accusative case, only the articles for masculine words change! The definite and indefinite articles for feminine / neuter / plural are the same as the nominative case.
The direct object is acted upon or "used" by the subject:
(The flowers are acted upon, they don’t do anything on their own, and they don’t receive anything → direct object)
(The car is acted upon, it doesn’t do anything on its own, and it doesn’t receive anything → direct object)
In sentences that have just the subject and a single object, the object takes the accusative case - except when a preposition or verb requires another case.
Most verbs require the accusative case.
The prepositions "um," "durch," "ohne," "bis," "für," and "gegen" can only be followed by nouns in the accusative case.
Prepositions don’t care if it’s a direct or indirect object, or what case the verb requires. If you have a preposition in front of a noun, the preposition determines the case. Always.
If you have a preposition in front of a noun, none of the other rules apply anymore. It doesn’t matter if it’s a direct or indirect object, or if the verb requires a specific case! All that matters is the preposition.
The preposition determines the case, ALWAYS!