The rule is this: For one-syllable words, you add -er for the comparative and -est for the superlative, e.g. biggest, oldest, hottest. For words of three or more syllables, you use 'more' for the comparative and 'most' for the superlative e.g. most beautiful, most dangerous. You can't say 'beautifulest', I'm afraid. A small child might say that, but it isn't correct. For words of two syllables, it depends on the type of word it is. Two-syllable adjectives which are verb participles use 'more' and 'most', e.g. most excited, most tiring, most boring. Two-syllable adjectives ending in 'y' add 'er' and 'est' e.g., easiest, happiest Other two-syllable adjectives, like 'common', can use either form. There is no difference in meaning between 'commonest' and 'most common'. We tend to choose the one that sounds most natural in a particular sentence.