在上古漢語裡，承接副詞「乃」可用於連貫複句、因果複句、條件複句的後一分句，表示較為寬鬆的事理關係，而可以根據語境的差異，翻譯為白話的「就」、「才」、「又」。此外，「乃」也再由承接副詞發展為轉折副詞、情態副詞。「乃」從承接副詞到轉折副詞，是因為承接副詞「乃」表示較為寬鬆的事理關係，可以出現在「兩件事的先、後發生可能違反說話者預期」的語境裡，並在「語境吸收」的作用下產生演變。「乃」從轉折副詞到情態副詞，是受到「主觀化」的趨勢影響，在脫離複句語境後，一方面在敘事句中由轉折副詞演變為意外義情態副詞，另一方面也在判斷句中由轉折副詞演變為辯白或申明義情態副詞。

In Old Chinese, the consecutive adverb “nǎi”（乃）can be used in compound complex sentences, causal complex sentences, and conditional complex sentences to express the broad relationship of events in the second clauses. Base on the context, it can be translated to “jiù”（就）, “cái”（才）, or “yòu”（又）in Mandarin. Besides, “nǎi” （乃）had been developing from consecutive adverb to adversative adverb since consecutive adverb “nǎi”（乃）that expresses the broad relationship of events, can be used in the context of the former and the latter events probably violate the expectation of the speaker. Furthermore, the development was formed by the absorption of context. The development of “nǎi”（乃）from adversative adverb to modal adverb was affected by the subjectification. After it broke away from the context of complex sentences, on the one hand, the adversative adverb transformed into the modal adverb, which expresses “unexpectedly” in narrative sentences. On the other hand, the adversative adverb “nǎi”（乃）became modal adverb, which expresses “turned out to be” or “indeed” in determinative sentences.