"We need a cupboard."
Translation:Wir brauchen einen Schrank.
DonFuchs1 -- that's the way the German language works shrugs Take it or leave it, really. I'm sure there are rules and phrases in your own native tongue that seem like 'nonsense' to learners. In German, you will need to contend with the fact that we have cases and aren't afraid to use them. ein can turn to eines, einem or einen, depending on which case the accompanying noun is used in.
Because the word Schrank is grammatically masculine, and it's the direct object of the verb brauchen (to need), so it's in the accusative case.
So you need the masculine accusative form einen before it.
A = ein?
"a" can be ein, eine, einer, einem, einen in German -- depending on the gender and case of the noun that it accompanies.