The mental picture I had of an elephant sipping on tea from a teacup is truly amazing!
Granted it did sound strange at first. But I just finished watching a video of an Indian elephant drinking a liter and a half of tea at his favorite tea shop and no longer think it is at all odd.
I think the correct answer should be "One elephant drinks tea". The English meaning of "An elephant drinks tea" is the same as "All elephants drink tea"; which should then be expressed via the plural construction.
They aren't! You just have to learn them. It is a shame that Duolingo doesn't give that information in their "hover" definitions. You have to pick it up from the way verbs are used with each noun. Or look the nouns up in a Hindi dictionary to find out for yourself.