Whilst providing German tuition, students often ask me what the Accusative and the Dative is.
Accusative and Dative are cases and they tell the function of nouns in a sentence. Many languages use cases, including English. In German, the direct object requires the Accusative case and the indirect object requires the Dative.
Consider the following example: The dog is biting the man. The dog is the subject (doing the biting), the man is the direct object; he receives the action of the verb (he is being bitten). Then the man kicks the dog. Here the man is the subject, and the dog is the direct object (he is being kicked). If you are replacing nouns with pronouns, then you can see the Accusative case in English: He is biting him. Him being the Accusative.
Now the indirect object comes into the equation: The man is going to the shop and is buying the dog a bone. The man is the subject, the bone is the direct object (it is being bought) and the dog is the indirect object (he receives the bone). Again, you can see the Dative case in English when replacing nouns with pronouns: He is buying him a bone. In this scenario, him is the Dative. In modern English, the indirect object is often distinguished by the use of prepositions, for example: He is buying a bone for the dog.
The German languages not only decline pronouns, but also articles, adjectives and sometimes even the noun itself.
Lets have a look at the above example in German: Der Mann kauft dem Hund einen Knochen.
der Mann = subject, dem Hund = indirect obkect (Dative), einen Kochen + direct object (Accusative)
Der Hund is changing to dem Hund and ein Knochen is changing to einen Knochen.
Apart from indicating the function of a noun, cases are also required after prepositions.