有些需要双写最后一个辅音字母在加-er, -est。如“胖瘦大热红伤湿”等。

　　fat—fatter, fattest;thin—thinner, thinnest;

　　big—bigger, biggest; hot—hotter, hottest;

　　red—redder, reddest; sad—sadder, saddestwet—wetter, wettest

特殊的形容词比较级和最高级 - 百度文库 (baidu.com)

一、少数单音节词前面加 more-, most- 构成比较级和最高级
tired ---- more tired , most tired
fond ----- more fond , most fond
glad ----- more glad , most glad
bored ---- more bored , most bored
pleased---- more pleased , most pleased
二、不规则变化
good /well------- better ,best
bad/badly/ill------ worse , worst
many/much-------more , most
little ------ less , least
far ---- farther, farthes / firther , furthest
old ---- older , oldest (GA)
    ---- older , oldest / elder , eldest (GB)
三、下列形容词和副词的比较级和最高级有两种形式
cruel----- crueler,  cruelest /more cruel , most cruel
strict---- stricter , strictest /more strict , most strict
often----- oftener , oftenest / more often , most often
friendly------ friendlier , friendliest / more friendly , most friendly
clever----- cleverer, cleverest /  more clever , most clever