"Cats are animals."
Translation:Οι γάτες είναι ζώα.
"Γάτες είναι ζώα" and it was counted as wrong. Must you include 'Οι' in this case? Why?
Because you need to be specific. Greek and English have different rules for this case. While in English the article is ommited, here the article is necessary. You're talking specifically about cats, generally being animals. It might sound a bit odd, but that's the logic behind it. That's why the article can be ommited in ζώα but not in γάτες. Also, you can't ommit the article in such a sentence, at the beginning of it. ^.^
I might be wrong but in a previous answer, "ducks are birds" didn't need articles?
This the sentence we have with "ducks". "Οι πάπιες είναι πουλιά." it does need the article.
Well, technically, τα γατιά could be kittens, both female and male in gender. Οι γάτες are female adult cats, while οι γάτοι are male adult cats. ^.^
(Although, keep in mind that these two (γάτες-γατιά, even γάτα-γάτος-γατί) are often interchangeable. One may have a male cat but still refer to it as "η γάτα μου".)