We use –
er and –
est to form most comparatives and superlatives.
clearer, clearest
We use ‘more’ and ‘most’ when an adjective has more than one syllable (except for two syllable words ending in ‘y’ when we use –
ier or –
iest).
more beautiful, most beautiful
happier, happiest
On the other hand, with adverbs that end in ‘ly’ we must use ‘more’ to form the comparative and ‘most’ to form the superlative.
more clearly, most clearly
I can’t make it any clearer than that.