Comparative adjectives are used to compare two (and only two) things.
They are used with the word THAN.

STRUCTURE

Subject   + BE + Adjective   + ER            + Than
Subject   + BE + More         + Adjective   + Than

Examples:
The car is bigger than the bicycle.
The dress is more expensive than the pants

How do you know when to add ER or when to use MORE?
Simply follow this rule:

Example Sentences:

The shoes are more expensive than the boots.
The boy is taller than his sister.
My house is smaller than your house.
The book is more interesting than the movie.

Practice comparative adjectives with this test:

Comparative Adjectives Test

Or, try this reading:

Comparative Adjective Reading