Introducing a complete question. / Introducing a verb phrase (bare infinitive clause). / Introducing a noun or other phrase. / (relative) For which cause, reason, or purpose.

Reason.

(dated or literary) An exclamation used to express pleasant or unpleasant mild surprise, indignation, or impatience.

(UK, dialect, archaic) A young heifer.

Contraction of why is. / Contraction of why has. / Contraction of why does.

Alternative spelling of whys plural of why

Why is that not true? / Used to state that one has no objection to and approves of a proposal. / Used to introduce a proposal.

plural of why

(obsolete) A violent and peremptory procedure without any assigned reason; a sudden conclusive happening.

(obsolete except non-native speakers' English) For what reason; why.