Demonstrative adjectives

Use:

This:                       Use this to talk about something which is near you.
                                                How much is this bag?

These:                  Use these to talk about two or more things which are nearyou.                                                
                                                Are these your keys?

That:                      Use that to talk about one thing which is far from you.
                                                Is that your house?

Those:                  Use those to talk about two or more things which are far from you.
                                                Who are those children over there?
                                                
Use This, That, These and Those:

• Before the verb be.

                This is my sister / Those are my children.

• Before a noun:

                That cake looks delicious! These bags are expensive!

Make questions this way:
                Is this / that your sister?               
                Are these / those your children?

You can contract is after that. 
                That is my dad.                                 =>           That’s my dad.
But do not contract is after this.
                This’s my dad.                                 =>           This is my dad.
Do not contract are after These / Those.
                
Common mistakes
Always use a verb in sentences with this, that, these and those.
This lovely bag.                                                =>           This is a lovely bag / This bag is lovely.

Simple adjectives
Use:
Simple adjectives are describing words.

• They can go before nouns.

                That’s a nice car.

• They can also follow the verbs be, look, feel, smell, sound and taste.

This food looks nice.  It smells delicious! The cake is lovely!