We use demonstrative adjectives when we want to point
(demonstrate) to a particular person or thing:
This car is mine.
That book is yours.
These children are ours.
Those people are Russian.
Demonstrative adjectives go in front of the noun that they point
to.
Russian makes no distinction between demonstrative adjectives
and pronouns. They are used as adjectives when they are in front of
a noun, and as pronouns when they replace a noun.
In Russian, demonstrative adjectives (and pronouns) agree in
gender, number and case with the noun they modify or replace.
Demonstrative adjectives answer the questions "which one" and
"which ones".
The demonstrative adjectives in Russian are:
gender
adjective
example
masculine
этот
Этот дом мой.
feminine
эта
Эта газета моя.
neuter
это
Это письмо твоё.
plural
эти
Эти дети наши.
It is very easy to confuse the word это, which means "this/that is"
and "these/those are", with the neuter form of the demonstrative
adjective, because they look the same.
The word это never changes its form, and its English translation always includes some form of the verb 'to be' :
This is my house vs. This house is mine.
These are our books vs. These books are ours.
The sentences on the left will use the word это. The sentences
on the right will use a demonstrative adjective, and will agree in
gender and number with the following noun.