"बिल्ली जानवर होती है ।"

Translation: A cat is an animal.

[shobana1256]

Why unnecessarily use "A" here "cat is an animal" is the correct sentence

[OJFord]

You do need an article like 'a' or 'the' in English. 'Cat is an animal' (with a capital C for a proper noun Cat) would refer to a cat (presumably!) named 'Cat'.

Alternatively you could make a general statement about plural cats: 'Cats are animals'. (Which could also have the definite article 'the', but would then not be a general statement, but rather refer to a specific bunch of cats.)

[xyfell]

I see the difference in number between the following:

बिल्ली जानवर होती है
and
बिल्लियां जानवर होती हैं

But why does it matter? When would I use one where the other would be improper?

[vinay92]

They are interchangeable.

In English, we usually prefer the plural form 'Cats are animals' to make general statements but either the singular or plural form is acceptable in Hindi.

[OJFord]

Indeed, but in this and many other cases the plural form like 'cats are animals' is not accepted. (I have reported this one.)

[juhani.juurik]

Indeed this does not sound like proper English to me.