«Э́то о́чень до́брая ко́шка» would work too, if the cat is female.
I personally tend to use кот for cats unknown or irrelevant gender. But if you want to imitate Moscow speakers as much as possible, then probably you should use кошка for cats of unknown or irrelevant gender indeed.
Am I just hard of hearing, or is it the norm to drop consecutive vowels when speaking? This sentence sounds to me like "Эт' очень добрый кот". I've noticed a similar kind of thing in other examples too (but I am perfectly willing to accept this is due to my lack of experience in listening to Russian).
«До́брое» is used before neuter nouns, «до́брый» is used before masculine nouns.
That would be «Этот кот очень добр.» or «Эта кошка очень добрa.».
The ‘This’ in the noun phrase “This cat” is a demonstrative adjective, so it agrees in gender with a singular noun in Russian: masculine ‘этот’, feminine ‘эта’, neuter ‘это’. In contrast, the “This” in “This [one] is” is a demonstrative pronoun, which defaults to neuter ‘Это’ if the gender isn't already clear from a previous reference.
The ‘nice’ in the verb phrase “is very nice” is a predicate adjective, so it takes the “short” form in Russian, which agrees only with the gender of the referent: masculine ‘до́бр’, feminine ‘добра́’, neuter ‘до́бро’. In contrast, the ‘nice’ in the noun phrase “a very nice cat” is an attributive adjective, so it takes the long form, which agrees with both the gender and the case (nominative in this sentence) of the noun.
[Updated] Well, almost. On second thought, I can think of a few types of exceptions.
For adjectives whose predicate form would be confusable with nominal forms, the attributive form is always or almost always used instead of the predicate form. For example, the adjective «золото́й|золото́е|золота́я|золоты́е» = “gold[-colored], golden” derives from the noun «зо́лото», and the attributive forms are ordinarily used even as predicates, instead of what would be the short predicate forms «*зо́лот|*золото́|*золота́|*золоты́». Another example is «голубо́й|голубо́е|голуба́я|голубы́е» = “blue”, derived from «голубь» = “dove”, whose predicate forms would be «*го́луб|*голубо́|*голуба́|*голубы́».
Ordinal numerals such as «пе́рвый|пе́рвое|пе́рвая|пе́рвые» = “first”, «второ́й|второ́е|втора́я|вторы́е» = “second” have no short predicate forms; the attributive form is always used.
Possessive adjectives such as «мо́й|моё|моя́|мои́» = “my” that look like attributive adjectives likewise have no short predicate forms; the attributive form is always used.
Well I am not a native, or on the Duolingo team, but I shall venture to answer your question regardless, as I think I can explain this correctly, and that way you will receive a quicker reply…
“Это очень добрый кот” translates directly as “This is a very nice (tom)cat.” Now, in English I guess (excepting sentence emphasis) “This (tom)cat is very nice” has an identical meaning, so, in theory, this is an acceptable translation.
However (and this is a BIG “however”), the point of the Duolingo course is to help us to learn the Russian language, and one of the first things that the course attempts to explain is the word “Это” and its use as a set phrase that doesn’t decline (meaning “This is”) and as a pronoun (meaning “this”) that declines in the normal way. Given that in this phrase the word “кот” is masculine, the use of “Это” instead of “Этот” shows clearly that “Это” here is being used as a set phrase (meaning “This is”), and as such, allowing the translation “This cat is very nice” (which does not contain the phrase "This is") would be to allow the learner to get away with what is very likely an error in their understanding.
Does that make sense? If you are struggling with the distinction between “Это” as a set phrase and as a pronoun, then there is a discussion board on the issue that I found very helpful, and I will gladly point you in its direction.
"Хороший". Also, you could say "Молодец!" (regardless of the gender).
the English 'This is' and 'These are' are usually translated into Russian as Это... The Indeclinable это.
The demonstrative pronoun этот (эта, это, эти) is used as a noun modifier. Compare the following statements. Pay attention to the intonation:
Это новый дом. This is a new house. Indeclinable это.
Этот дом новый. This house is new. Demonstrative pronoun. (masculine). http://www.russianforeveryone.com/Rufe/Lessons/Course1/Grammar/GramUnit6/GramUnit6_3.htm
Это очень добрый кот = This is a very nice cat. Этот кот очень добрый. = This cat is very nice.
They look similar, but это is a free-standing pronoun (it doesn't modify a word), and этот is a modifier for a masculine noun.