why
n.n. the word you use to ask about the reason for something. It helps you understand the cause of an action or a situation.
n. used to introduce a question or a clause concerning the reason, cause, or purpose of an event. Functions as an interrogative or a relative adverb depending on the syntactic structure.
Why are you late for the meeting?
The manager asked why the project was behind schedule and requested a detailed report by Friday.
Philosophers have long debated why humans seek meaning in a universe that often appears indifferent to their individual struggles and triumphs.
From Middle English why, from Old English hwȳ (“why”), from Proto-Germanic hwī (“by what, how”), from Proto-Indo-European kʷey, instrumental case of kʷís (“who”), kʷid (“what”). Cognate with Old Saxon hwī (“why”), hwiu (“how; why”), Middle High German wiu (“how, why”), archaic Danish and Norwegian Bokmål hvi (“why”), Norwegian Nynorsk kvi (“why”), Swedish vi (“why”), Faroese and Icelandic hví (“why”), Latin quī (“why”), Doric Greek πεῖ (peî, “where”), Ukrainian чи (čy, “if”), Polish czy, Czech či (“or”), Serbo-Croatian či (“if”). Compare Old English þȳ (“because, since, on that account, therefore, then”, literally “by that, for that”). See thy.
Commonly introduces direct questions or indirect interrogative clauses; can also function as a relative adverb after the noun 'reason'.
I don't know why did he leaveI don't know why he leftIn indirect questions, the word order should be subject-verb, not the auxiliary-subject order used in direct questions.