The word "это" appears in texts quite often. But sometimes it stays the same no matter what word comes after it, or sometimes it's declined according to the word it's reflecting. For example:
Это грамматика. У тебя есть эта грамматика?
As you can see, это comes in two different forms for the same word. I come across this situation rather often and wonder when это should be declined.
Это is not declined when it means "this is". Or, for that matter, "these are"; you'd still say это книги "these/they are books", because эти книги would be an incomplete statement strictly meaning "these books".
By extension, это is not declined in its role as quasi-copula in statements of definition or explanation: Пантера — это чёрный леопард или ягуар.
And as you might guess, it gets ambiguous when you have a singular neuter noun: это зеркало can be both "this is/it's a mirror" and "this mirror". You'll have to infer from context which one it is.
Possibly useful comparison in English: "it" generally denotes a neuter noun. However if you say, "Who's that? It's my father", this does not mean my father is neuter.
Anyway that's how my first ever Russian teacher explained it, many years ago :)
This link to Wiktionary should help you. Это is the neuter form of этот therefore it shares the same declension as the latter with the exception of the nominative and accusative cases, where это is not declined at all.
