为什么 (wèi shén me) is the standard Mandarin word for 'why', used to ask for reasons or explanations. It functions as an interrogative adverb and always appears at the beginning of a question — never at the end, unlike English tag questions. The character 为 means 'for' or 'because of', 什 is a bound morpheme with no independent meaning here, and 么 is a grammatical particle that combines with 什 to form the interrogative pronoun 'what'; together, 什么 means 'what', and 为什么 literally asks 'for what [reason]?', hence 'why'.

This term is neutral in register and appropriate in both spoken and written Chinese across all contexts — from children asking about daily routines to adults discussing complex topics. It can introduce yes/no questions (e.g., 你为什么不去？) or be paired with other question words like 谁, 哪里, or 怎么. Unlike some English 'why' questions, 为什么 does not carry inherent judgment — tone and context determine whether it sounds curious, concerned, or critical.