eisiau is a noun meaning both a need or a want/lack, so it can be used in both senses. angen is perhaps more commonly used for 'a need'. Using rhaid is an option, too, in some senses.
- dw i eisiau prynu moron - I want/need to buy some carrots
- dyn ni angen moron - we need some carrots
- (mae) rhaid i fi brynu moron - I must/have to buy some carrots
If moron is singular and plural, how do you know which is correct in this sentence? I said, "I want to wash the carrot" and it was marked wrong.
moron is the plural. moronen is the singular.
This common with things that usually come as many together, such as vegetables and trees:
- derw, deri - oaks. derwen - an oak-tree.
- helyg - willows. helygen - a willow-tree.
- pysgod - fish. pysgodyn - a fish.
- tatws - potatoes. taten - a potato.
- pannas - parsnips. panasen - a parsnip.
- ffa - beans. ffeuen - a bean.
- cnau - nuts. cneuen - a nut.