“sad” in Japanese is 悲, read hi, kana (ひ, かな). It is a Grade 3 Kyōiku kanji, taught in year 3 of Japanese elementary school, and appears in words such as 悲しい (sad).
What does 悲 mean?
友だちと別れて悲しい。
Same sound, different character — the classic trap. Compare them side by side: