is it right that, when a word has more than one syllable , one should use "more" in front of it in order to express the increase of intensity/size/amount?
yes,but only if it doesn't end in -y , because then it takes the -ier and -iest patterns,just like
pretty prettier prettiest
happy happier happiest
dirty dirtier dirtiest
if there are 2 syllable in "simple" why do people say "simpler" instead of "more simple"?
because , as a two-syllable adjective ending in an unstressed vowel, simple takes the -er/-est patterns (just like clever)
hope this helps.
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