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https://english.stackexchange.com/questions/162576…
Why: a relative adverb, a conjunction ... or both?
When, where, why, and how belong, however, to a different special class of words, with equally special grammar. In fact, they constitute one subclass of this special class. Most of the other wh -words (what, which, who) refer to nouns, so they're called pronouns -- either interrogative pronouns (because they're used to introduce question clauses)