The word "this" is a demonstrative pronoun when it takes the place of a noun in a sentence.
The word "this" is an adjective when placed before a noun to describe that noun.
In the sentence, " This is a very beautiful painting.", the pronoun "this" is the subject of the sentence.
A demonstrative pronoun takes the place of a noun indicating near or far in place or time. The antecedent (noun) is indicated by a gesture or by previous mention.
The demonstrative pronouns are: this, that, these, those.
The demonstrative pronouns function as adjectives when used to describe a noun, as in the sentence, " This picture is very beautiful."