According to the website dicionário Cambridge, demonstratives are word that show or point at something.
Demonstratives: are determiners that points to a noun in particular ( an object, a person, an event ). There are four demonstratives to describe distance and number.
This & These: short distances
This and These point to near objects.
This is used before singular nouns:
This computer is modern.
This party is awesome.
This pie is delicious.
What is this?
These is used before plural nouns:
These girls is are the best.
These fruits are fresh.
These houses are wonderful.
Are those clothes clean?
That & Those: long distances
We use That and Those to point or show something far from us.
That is used before singular nouns:
That ship is white.
That tall bay is Ana’s brother.
That man is my new teacher.
Who is that?
Those is used before plural nouns:
Those glasses are new.
Those people over there are my friends.
Those pictures are expensive.
Past X Present
When we talk about actions in a near period of time we use: This and These:
This is the best time of my life.
These days are really hot.
This year is good.
If the action is in the past we use That and Those:
That night was great.
Those years were great.
Those times are over.
We use demonstrative to introduce people
This is Sandra.
This is my sister Camilla.
This is my boss, Hector.
These are my friends. This is Susan and this is Brian.