Quote Originally Posted by Alex80 View Post
P.S.

Also, we can see this in other examples:
"Это была злая старуха"
vs
"Эта старуха была злая" - "This old woman was angy".

It is the same thing. "Это" and "эта" are related to different words.
I am not sure about exact rule...
As Hoax mentioned in case of unchanged это it's actually a subject therefore it doesn't change and stand in nominative. When это changes it setrves a function of a pointer and relates to a word it describes, just like any adjective does.

Examine this:

It's them! Them is plural but "it is" structure is not.

Same thin happens with это in Russian.