本文旨在探討多義情態詞「要」諸多用法的來源。研究發現在兩漢之交， 「要」發展為表「必定」義的情態副詞，兼屬認識情態及義務情態兩大範疇。 其來源可能有二：一為表「主要、重要」義的用法；另一為「要之」之省。「要 之」本是出現於謂詞組之前，表「總其要點」的述賓結構，由於「之」的刪 略，導致動詞「要」與後頭的謂詞組進行重新分析，「要」由此變成情態副詞。 其中表「必然」一類沿用至今，仍保有副詞的特性；表「必要」一類，則於 六朝時期逐漸發展為情態動詞，此句法功能上的轉變或許肇因於「要」與義 務情態動詞「當」經常性的連用。唐以後，表「意願」及標誌「近將來時」 為「要」的兩個新興用法，它們與「要」先前的語義皆無連繫，可能是因為 「欲」另產生類似於iau 的又讀，借用同音詞「要」的外殼體現，使得「要」 就此一併接收其兩種情態意涵。

This essay looks into the various functions of the modal yao 要. By the Eastern Han dynasty, yao was already a modal adverb denoting epistemic and deontic ‘must.’ Such a function derives from [1] constructions where yao meant ‘main’ or ‘important,’ and [2] constructions where yao zhi (要之 to sum it up) came before another VP. With the later omission of zhi, the remaining ‘yaoVerb VP’ therefore came to be reanalyzed as ‘yaoModal Adverb VP.’ The epistemic modal adverb yao is still in use, while the deontic modal adverb yao evolved during the Six Dynasties into a deontic modal verb, owing to its frequent co-occurrence with the deontic modal verb dang 當. From the Tang dynasty onwards, yao also became capable of expressing volition and the proximate future. Although these latter two functions seem untraceable, they were in fact taken on from yu 欲, whose similar alternative pronunciation (i.e. [iau]) facilitated their absorption into yao.