1. 

In Comparison of two qualities of the same person or things -er form is not used.

He is more wise than brave.

2. Comparative Adjectives followed to  not than 

eg

Instead of prefer than, prefer to 

3.

-Elder or eldest are used for persons of the same family

-Older, oldest for other family. 

4. Some adjectives : ideas, like, perfect, round, entire, extreme, full ) do not agrees comparison

eg. It is a unique ideas.

5. No double comparatives or superlatives . 

6. Cardinal and ordinal are used together 

The four first boys will be given a chance.

or 

The first four boys will be given a chance.