Both flag the unexpected, but differently. 居然 = "surprisingly, to one's astonishment" — it simply marks the event as astonishing, often letting the speaker tack on a comment. 反而 = "on the contrary" — it marks a reversal, the opposite of what should have followed, and pairs with 不但没 / 不但不 …,反而 …, where 居然 can't.
構文: 居然 + astonishing fact (+ comment) · 不但没 / 不但不 …,反而 …
he他so这么young小astonishingly居然can能recite背hundreds of几百首Tang poems唐诗truly,真remarkable了不起。
So young, yet he can recite hundreds of Tang poems — truly remarkable.
after taking吃了these这些pills药he,他not only不但didn't没get better好but instead,反而got even更worse严重了。
After taking these pills, not only did he not improve — he got worse.
** remember : 居然 = "astonishingly!" (just marks surprise, + optional comment); 反而 = "on the contrary" (a reversal). Only 反而 completes 不但没 / 不但不 …,反而 …. 居然 is amazement; 反而 is a flip.
Watch out
- A surprise that isn't a reversal takes 居然; the "opposite happened" twist takes 反而.