"I have a plate with rice."
Translation:Ich habe einen Teller mit Reis.
This sentence is in the accusative case, not nominative. In nominative, male = der/ein, neutral = das/ein and female/plural = die/eine. However, in accusative case, the male turns to den/einen. Certain verbs turn a sentence accusative, including "haben" (to have). Nehmen (to take), bestellen (to order), brauchen (to need) and others are also accusative.