Possible Duplicate:
“More clear” vs “Clearer”, when to use “more” instead of “-er”
Are there any rules for which words are allowed to have -er and -est endings? Being a native speaker, I do it by ear, but what distinguishes free/freer/freest, calm/calmer/calmest, and speedy/speedier/speediest from watchful/*watchfuller/*watchfullest or creative/*creativer/*creativest ?