Thread: Это

Everything written above is correct. Especially I liked the link, it provides a very detailed answer. I would just add some simple explanation of my own
OK, there are two "это"s: "это" used as a noun, and "это" used as an adjective. Both of them have the same meaning ("this"), but they differ in their syntactic usage.

When "это" substitutes a noun, it is used independently. In this case, "это" is always neuter and singular. It is often called "unchangeable это", but I would say it is not an accurate statement. "Это"-noun never changes its gender and number, but it does change cases.

Consider the following examples:
Это стол. (Стол is masculine!) – This/that/it is a table.
Это книга. (Книга is feminine!) – This/that/it is a book.
Это окно. (Окно is neuter!) – This/that/it is a window.
Это столы. (Noun in plural!) – These/Those/they are tables.
Это книги. (Noun in plural!) – These/Those/they are books.
Это окна. (Noun in plural!) – These/Those/they are windows.

All the examples above are in Nominative.

However, "это"-noun can be used in any other grammar case:
От этого зависит наше будущее. (Это in Genitive) – Our future depends on this/that/it.
К этому надо привыкнуть. (Это in Dative) – One has to get used to this/that/it.
Я это знаю. (Это in Accusative – the same form as the Nominative) – I know this/that/it.
Я не умею этим пользоваться. (Это in Instrumental) – I do not know how to use this/that/it.
Я ему об этом уже рассказал. (Это in Prepositional) – I have already told him about this/that/it.

When "это" is used to describe a noun, it behaves like an adjective, and it is not independent (makes no sense without a noun it modifies). In this case, "это" agrees with the noun in gender, number and case.
Compare below:
Это стол. (Стол is masculine!) – This/that/it is a table.
but:
Этот стол большой. – This table is big.

Это книга. (Книга is feminine!) – This/that/it is a book.
but
Эта книга большая. – This book is big.

Это окно. (Окно is neuter!) – This/that/it is a window.
but
Это окно большое. – This window is big.
Note: here the both forms are the same, because of the neuter gender of "окно".

Это столы. (Noun in plural!) – These/Those/they are tables.
but
Эти столы большие. – These tables are big.
etc. (all the examples above are in Nominative).

Now with the "oblique" cases (other than Nominative):

Genitive: Я не могу без этого стола. Я не могу без этой книги. Я не могу без этого окна. Я не могу без этих вещей. – I cannot do without this table/this book/this window/these things.
Dative: Подойди к этому столу. Подойди к этой книге. Подойди к этому окну. Подойди к этим вещам. – Come closer to …
Accusative: Я вижу этот стол. Я вижу эту книгу. Я вижу это окно. Я вижу эти вещи. – I see …
Instrumental: Я доволен этим столом. Я доволен этой книгой. Я доволен этим окном. Я доволен этими вещами. – I am satisfied with …
Prepositional: Я говорю об этом столе. Я говорю об этой книге. Я говорю об этом окне. Я говорю об этих вещах. – I speak about …

It's interesting to note that some languages even have completely different words for "this"-noun and "this"-adjective. E.g., in Japanese "this"-noun is KORE / SORE / ARE (depending on whether it is close to the speaker, or close to the listener, or far away from both). But "this"-adjective is KONO / SONO / ANO.

There are some good explanations above. Just wanted to add that "это" is unchangeable when it means "there is/there are". The source of all confusion is because unchangeable "это" has the same form as neuter singular (nominative and accusative) of the demonstrative pronoun "этот" (this, this one, that).

это - doesn't change form

этот - declensions (looks like это in Neut.sg.)

	Masc. sg.	Fem. sg.	Neut. sg.	Plural
nominative	э́тот	э́та	э́то	э́ти
genitive	э́того	э́той	э́того	э́тих
dative	э́тому	э́той	э́тому	э́тим
accusative	э́тот, э́того	э́ту	э́то	э́ти, э́тих
instrumental	э́тим	э́той, э́тою	э́тим	э́тими
prepositional	об э́том	об э́той	об э́том	об э́тих

In the original Rosetta's example, Это стадион means "This is a stadium" because the woman points to the picture of the stadium.
The verb "to be" is not used in Russian in the present tense, you can sometimes see a dash after это instead of the verb "to be". Это - стадион.

The woman could also give a definition of the stadium like this:
Стадион - это место, где много народу.
Stadium is a place where there is a lot of people.