何故 is a formal, literary adverb meaning 'for what reason?' or 'why?'. It combines 何 (hé), an interrogative pronoun meaning 'what' or 'which', and 故 (gù), meaning 'reason', 'cause', or 'hence'. Together, they literally mean 'what cause?', forming a concise, classical expression for asking about motivation or justification. It appears frequently in written Chinese—such as essays, historical texts, and formal speeches—but is rare in casual spoken conversation, where 为什么 (wèi shén me) is far more common.

Although grammatically an adverb, 何故 functions like an interrogative phrase that typically begins a sentence or clause, often followed by a verb or descriptive predicate. It carries a tone of solemnity, mild surprise, or dignified inquiry—making it suitable for expressing disappointment, curiosity, or moral questioning. Learners should note that it is not used with auxiliary verbs like 会 or 能, nor does it take aspect markers like 了 or 过; it stands independently before the main verb.