张道真《实用英语语法》有具体说明

1.以ow,er,le结尾的和个别其他双音节词，可以加词尾的办法构成比较级和最高级：

narrow,narrower, narrowest;

simple, simpler, simplest;

common, commoner, commonest;

clever, cleverer, cleverest;

quiet, quieter, quietest;

polite, politer, politest;

2.以辅音字母+y结尾的词，变y为i，再加词尾。

busy, busier, busiest;

heavy, heavier, heaviest;

所以：happy, happier, happiest；early, earlier, ealiest;