The word than indicates that this is a comparative, so we can't use most difficult or as difficult. Difficult is a three-syllable adjective, so it doesn't change its form in the comparative. We should therefore use more difficult.
Comparative and superlative
Comparative vs superlative: comparative compares two things (taller than); superlative picks the extreme from three or more (the tallest). If there are only two options, never use the superlative — the taller of the two, not the tallest of the two.
Comparatives use -er or more; superlatives use -est or most. Short adjectives take suffixes; longer ones take more/most. A small irregular set (good/better/best) follows no pattern.
Diagnostic: how many items are being compared? Two → comparative. Three+ → superlative. Also: never double up (more better is always wrong).
Adjective
Adjective vs adverb: both describe things, but adjectives attach to nouns while adverbs attach to verbs. A quick answer (adjective → noun) vs answered quickly (adverb → verb).
An adjective modifies a noun or pronoun — telling you what kind, which one, or how many: a red car, something useful, three heavy boxes.
Diagnostic test: if the word describes a thing or person, use the adjective form. If it describes an action, you need the adverb (-ly) form instead.
B2 | Upper Intermediate
B2 vs C1: B2 means effective communication on complex topics with some effort. C1 means effortless fluency with precise register control. If you can argue a point but still reach for words and make structural slips under pressure, you're B2.
B2 is the upper-intermediate CEFR level: mixed conditionals, complex passives, reported speech with backshift, participle clauses, and sustained written argument.
Diagnostic: does your writing read as "competent non-native" or "could be native"? The former → B2. The latter → C1.
Hard
Hard vs Medium: Medium tests one rule with realistic distractors. Hard tests interacting rules, edge cases, or context-dependent answers where multiple options seem correct until you think deeply. If you're scoring 80%+ on Medium, try Hard to find your real gaps.
The Hard tag filters for B2+ challenges with layered difficulty: rule interactions, subtle distractors, and contexts that demand genuine grammatical reasoning.
Diagnostic: if Hard questions feel impossible, drop to Medium and master the individual rules first. Hard assumes you already know each rule — it tests whether you can apply them together.