"I have a plate with rice."
Translation:Ich habe einen Teller mit Reis.
when should it be einen vs ein or eine? I haven't used einen before this....
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.
I used the wrong form of ein, so it just corrected that and didn't offer other acceptable answers, but would 'Ich habe einen Teller Reis' also work, as with 'bowl (of) soup' where the 'of' is left out in German?