Short adjectives add -er, but long ones use 'more'. For a three-syllable word like 'difficult', which comparative is correct? 'This problem is ___ than the last one.'

English adjectives of three or more syllables, like "difficult," form their comparative with "more" rather than an -er ending, so "more difficult" is the correct, natural-sounding form, while adding -er directly to a long word produces something no native speaker would actually say.

"Difficulter" simply isn't a word English speakers use — the language reserves -er endings mainly for short, one- or two-syllable adjectives — and "most difficult" is the superlative form, used for comparing three or more things rather than just two.

The rough rule of thumb — short adjectives take -er, long ones take "more" — has exceptions and gray areas around two-syllable words, which is why comparatives remain one of the trickier corners of English grammar for learners to master.