Demonstratives

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.

Expressions with demonstratives

• This is it!
• That’s it!
• Tha’s all!