The negator Bié (別) is the shortened form for bùyào (不要), which is primarily used in unrealized situations. In the sentence 你昨天為什麼別去參加婚禮 (nǐ zuó tiān wéi shí me bié qù cān jiā hūn lǐ) , Bié (別) is used in an actualized event (attended the wedding) at a past time (昨天), violating its functional restriction.
