"The hare is an animal and a mammal"
Translation:Ο λαγός είναι ζώο και θηλαστικό
As a rule of thumb, the indefinite article in Greek can be omitted if there cannot be more than one of the following noun/attribute. A hare cannot be more than one animals or mammals, so the indefinite article can be omitted. But in a sentence like "I want a pair of shoes=Θέλω ένα ζευγάρι παπούτσια" it cannot be omitted because you may want more than one pair of shoes.
Would a good rule of thumb be that if the indefinite article can be translated as 'one' then it must be included? So for example in English you could say "I ate one carrot" instead of "I ate a carrot", but you wouldn't say "The carrot is one vegetable". So the article could be omitted in the second sentence but not in the first?