How I use the 4 declinations of German?, who can help me?
I know to use the nominative, but no the accusative, dative and genitive.
Nominative = The thing you are focusing on in a sentence. Usually it is the one doing the action. Eg: THE MAN is eating an apple (Der Mann isst einen Apfel)
Accusative = The thing that is having something done to it in a sentence. Eg: The man is eating AN APPLE (Der Mann isst einen Apfel)
Dative = A thing in the sentence which is involved but not Nom/Acc. Eg: The man gives an apple to THE WOMAN (Der Mann gibt der Frau einen Apfel)
Genitive = Used to show the owner of something. Eg: THE MAN’S apple (Der Apfel des Mannes)
To expand on this, Dative is usually the receiver of the accusative object.
I buy coffee - coffee is accusative. To add a dative component to the sentence, add a receiver of the coffee:
I buy you a coffee / I am buying a coffee for you - Ich kaufe dir einen Kaffee.
There are also other rules that dictate which ones to use. For example, the word "mit" (with) is ALWAYS followed by the dative. So is the verb "helfen" (to help) even if there is no accusative.
I'm slapping him - Ich schlag ihn. (accusative)
I'm helping him - Ich helfe ihm. (dative)
There are lists of these exceptions, and you'll find lessons categories in the German tree that slowly teach them to you.
If you're talking about something that is located in a certain spot, it's dative, but if it's something moving TO a location, then it's accusative.
He sets the lamp on the table. - Er stellt die Lampe auf den Tisch (accusative)
The lamp is on the table - Die Lampe ist auf dem Tisch. (dative)
I'm not sure if this helps learning it, but as a native German when I want to know whether something is nominative/genitive/dative/accusative then I use these questions on the sentence to find out:
nominative = 'wer?' = the answer to: 'who is doing it?' accusative = 'wen?' = the answer to: 'who do I see/eat/bring/meet/like/etc....'? dative = 'wem?' = the answer to: 'to/from whom? genitive = 'wessen?' = the answer to 'whose is it?'
(ie as a native I automatically know whether it's der/dessen/dem/den but I don't always remember what case that actually corresponds to if someone asks)
Those are cases. I suggest you read the tips and notes as you progress through the course, as it's difficult to attack them head on all at once.
But the gist of them, very simplified:
Nominative: Subject and subject predicate.
Accusative: Direct object (accusative object), object predicate
Dative: Indirect object (dative object)
Prepositions can cause either of the cases, except for Nominative.
As for declinations, they are related to the gender (class) and the singular/plural of the nouns.