We use Demonstrative Adjectives to talk about specific examples of a noun. These are a kind of determiner.
The demonstrative adjectives are:
Although these look exactly like the demonstrative pronouns, when placed in front of a noun they function as demonstrative adjectives.
We use them to talk about whether a noun is singular or plural, near or far.
singular – plural
near – this these
far – that those
When we talk about a singular or non-countable noun, we use this if the noun is close to us (in space or time), or that if the noun is far away (in space or time):
Look at this picture here.
Can you see that mountain over there in the distance?
I’m going on holiday this week.
That was a long time before I was born!
When we talk about a plural noun, we use these if the noun is close to us (in space or time), or those if the noun is far away (in space or time):
Look at all these pictures here.
Can you see those mountains?
I’ve been feeling quite ill these days.
I remember all those summer days when I was young.
Usage vs Speaker
Which word you use will depend on the speaker, who must use their own frame of reference. In this example the noun being talked about is the same but the two speakers use a different demonstrative adjective because one is close whilst the other is distant.
Determiners in English Grammar – a general look at determiners