The regular form of comparative adjectives (better, faster, smarter, worse, etc.) is made by replacing the ending with -ее. Some adjectives (including ones where the normal neuter ending is -ее instead of -ое and ones where the stem ends on a д к х т or г), however, have irregular forms. Here is a short list of them. I doubt this is all of them, but these are very common ones.
ранний - early
раньше - earlier (sooner)
поздний - late
позже - later
большой - large
больше - larger
маленький - small
меньше - smaller
краткий - short
кратче - shorter
долгий - long
дольше - longer
молодой - young
моложе - younger
старый - old
старше - older
далёкий - far
дальше - farther
близкий - near
ближе - nearer
хороший - good
лучше - better
плохой - bad
хуже - worse
дорогой - dear (precious, expensive)
дороже - dearer (more precious, more expensive)
дешёвый - worthless (cheap, modest)
дешевле - more worthless (cheaper, more modest)
I don't know why the ё disappears in the last one.