Demonstrative adjectives answer the question: Which one or ones? For example:

Which one do you like? Answer: That one.

Which shoes did you buy? Answer: These shoes.

Which house do you live in? Answer: This house.

Which flowers are the prettiest? Answer: Those flowers.

“This” and “that” are used with singular nouns. Examples: This meal is delicious. That cat is fat.

“These” and “those” are used with plural nouns. Examples: These shoes are too tight. Those socks are mine.

“This” and “these” are used to describe things that are close by. Examples: This dress I am wearing was cheap on the sale. These ice-cubes in my glass are melting fast.

“That” and “those” are used to describe things that are further away. Examples: I would like to buy a book on that shelf over there. Please pass me those scissors.