Rule reminder
The choice between 'di' and 'che' in comparisons is critical: use 'di' when comparing two DIFFERENT things, people, or quantities with the same adjective/verb. Use 'che' when comparing two DIFFERENT adjectives, verbs, or adverbs applied to the SAME thing/person. Also use 'che' before infinitives, prepositions, and conjunctions.
Questo esame è più difficile _____ quello dell'anno scorso.