What are demonstrative adjectives? In English, they are this/these, that/those. Demonstrative adjectives are used to point out people and things that are nouns. The demonstrative adjective must be in accordance with the noun, whether it is in plural or singular.
This/that = singular
I like this cat.
This guy really knows what he’s talking about.
She really wants to get to know that man.
I haven’t seen that lady before in my life.
These/those = plural
I’ve never seen those people before.
Can you please show me those books after you’ve read them?
I like this pencil but these pencils are much prettier.
These clients of ours are constantly ringing up.
Demonstrative adjectives vs. demonstrative pronouns
Demonstrative adjectives: point out or signal things and people, they act as an adjective only.
I like that dog of yours. (‘dog’ is the noun being pointed out)
Mary loves those friends of hers. (‘friends’ are the people being pointed out)
Demonstrative pronouns: take the exact same form as demonstrative adjectives (this/these, that/those), only that they do not modify or point out the noun. They stand alone as pronouns, therefore they can’t function as adjectives, but as pronouns.
This is so stupid. (‘this’ refers to whatever activity is stupid)
These are the worst I have met. (Here, ‘these’ could refer to the people he met)
I like these. (‘these’ refers to whatever he/she likes)
In all the above examples the demonstrative pronouns replace the nouns, therefore they cannot be demonstrative adjectives.