Questions & Answers about Lan vui hơn An.
The pattern is:
Subject + Adjective + hơn + Object-of-comparison
Here: Lan (subject) + vui (adjective) + hơn + An (object of comparison).
You replace hơn with kém … hơn, which means “to lack in comparison.” So:
Lan kém vui hơn An.
Literally: “Lan is less happy than An.”