The object of a verb goes into the accusative, and less changes than you might fear. Die, das and plural die stay exactly as they are. Only masculine der becomes den, and ein becomes einen. So ich reserviere ein Zimmer but ich nehme einen Schlüssel.
Examples
- Ich möchte ein Doppelzimmer reservieren.
- I would like to reserve a double room.
- Haben Sie den Schlüssel für Zimmer zwölf?
- Do you have the key for room twelve?
- Wir nehmen das Zimmer mit Frühstück.
- We will take the room with breakfast.
The mistake to avoid
Only the masculine changes - die and das look identical in the accusative.
Where this fits in
Travel & directions, German A2: Get around in a German-speaking city: take transport, ask the way, check into a hotel, and buy a ticket.