曷 is a classical interrogative ('why / how / when') largely replaced by 何 in modern Chinese. It survives mainly as a phonetic root, giving the sound in 喝 (to drink), 渴 (thirsty), 竭 (exhausted), and 歇 (to rest).
register
Literary and archaic. You will see 曷 in classical texts and in component breakdowns, not in everyday speech.
