形容词的比较级，用于比较两个物体。
形容词的最高级，用于比较多个物体。

单音节形容词，+ er / est

old – older – oldest
young – younger – youngest
long – longer – longest
short – shorter – shortest
bright – brighter – brightest
close – closer – closest

结尾是y的形容词，y改成i后，加er/est

pretty – prettier – prettiest
happy – happier - happiest

多音节形容词，前面加 more/most

respectable – more respectable – most respectable
beautiful – more beautiful – most beautiful
preferable – more preferable – most preferable
hardworking – more hardworking – most hardworking

一些特殊情况

good – better – best
bad – worse – worst
little – less – least
much (many) – more – most
far – further - furthest

than 通常用在比较句中

Amy is smarter than Betty.
Chad is stronger than Dan.
Greg is more diligent than his brother.
I have more apples than he.
She likes him more than me.

在'最什么'的形容词前，加the

Tom is the oldest man in town.
Paul is the tallest boy in the neighborhood.
That shade of blue is the most beautiful color.
This is the longest song that I have ever heard.

参考

http://www.talkenglish.com/grammar/comparative-superlative-adjectives.aspx