DEMONSTRATIVES: Explanation

Why do we use this and these?

We use this (singular) and these (plural):

1.  to talk about people or things near us:

This is a nice cup of tea.
Whose shoes are these?

We have lived in this house for twenty years.
Have you read all of these books?

2. to introduce people:

This is Janet.
These are my friends, John and Michael.

WARNING:
We don’t say These are John and Michael.
We say This is John and this is Michael.

Why do we use that and those?

We use that (singular) and those (plural):

- to talk about things that are not near us:

What’s that?
This is our house, and that’s Rebecca’s house over there.
Those are very expensive shoes.

Who lives in that house?
Who are those people?

- We also use that to refer back to something someone said or did 

• - I’ve got a new job.
  - That’s great.
• - I’m very tired.
  - Why is that?

taken from British Council