Rules for Comparative and Superlative adjectives

When do you simply and the -er (comparative) or -est (superlative) ending to the adjective, and when do you have to use more or most instead? 

Here are some charts from onestopenglish.com that show some of the basic rules (I have not provided a link to the site itself because you have to register before you can view material on this website:

Quizzes on Comparative and Superlative Adjectives

Click the first link below to take a quiz on comparative adjectives. The second link will take you to a similar quiz on the superlative form of adjectives.

For each quiz, you will see an empty box with a sentence underneath. Choose the adjective from the box at the right of the page and put it in the proper form (comparative for the first quiz and superlative for the second) to complete the sentence.

LINK: Comparative adjectives quiz

LINK: Superlative adjectives quiz