Who acts, what is affected, who receives it?

The nominative gives the subject: Der Nachbar kommt. Many verbs have an accusative direct object: Ich sehe den Nachbarn. Giving has a dative recipient and an accusative thing: Ich gebe dem Nachbarn den Schlüssel. Nachbar also adds n in these object forms; learn the useful complete phrases.

A preposition may choose the case

mit, zu, bei, von, aus, nach and seit take the dative. für and ohne take the accusative. With in, auf and related location prepositions, a location uses the dative, while a destination into or onto it uses the accusative. Movement within a place is still a location: Wir laufen im Park.

Ownership and short pronouns

Ownership words take the noun’s ending: mein Schlüssel, meinen Schlüssel, mit meinem Schlüssel. Compare masculine ihn as a direct object with ihm as a recipient: Ich sehe ihn, Ich gebe ihm den Schlüssel. Feminine uses sie and ihr. Polite object forms are Sie and Ihnen.

Keep negation clear

nicht negates an activity or description; kein replaces an indefinite article. Ich bleibe nicht in Berlin denies the location. Ich habe keinen Vertrag denies having a contract. The ordinary verb forms stay the same. A German negative sentence does not need a special negative verb form.