In english we use the suffix "er" to imply that something is comparatively more than something else. After looking through a list of Esperanto affixes I cannot find any equivalent. So for the sake of example, how would I say bigger? Or does the word big remain unchanged while we add the word than?

Kalantir:In english we use the suffix "er" to imply that something is comparatively more than something else. After looking through a list of Esperanto affixes I cannot find any equivalent. So for the sake of example, how would I say bigger? Or does the word big remain unchanged while we add the word than?

big = granda
bigger = pli granda
biggest = plej granda

small = malgranda
smaller = pli malgranda
smallest = plej malgranda

big = granda
less big = malpli granda
least big = malplej granda

Mi estas pli alta ol vi
I am taller than you

Li estas la plej juna el miaj filoj
He is the youngest of my sons

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harder = pli malfacila (more difficult) or pli dura / pli malmola (if you mean hard as firm or rigid; not easily dented, crushed, or pierced)
better = pli bona
faster = pli rapida
stronger = pli forta