Stacks a second fact on top of the first about the same subject — not only A, but also B; the two add up rather than cla
框 · Frame
不但 [clause A], 還/也 [clause B] (B adds to A, same subject)
觸 · Trigger
One thing is true, and a second thing piles on top of it.
序 · The move
1State the first fact and open it with 不但.Is there a real second item coming? If only one, drop the frame.
2Head the second clause with 還 or 也 and add the further fact.Does B add to A on the same subject, or does it turn against it? A turn wants 但是, not this.
3Keep one subject across both halves, or restate it in the second half.Same subject throughout? If B has a new subject, it often takes 而且 instead.
例 · Examples
1在超商不但not only可以can買東西buy things，還also; on top of that可以寄東西send things (mail)。
At a convenience store you can not only buy things, but also send packages.
不但…還/也… stacks upward (A holds, B adds); 雖然…但是… concedes then turns (A holds, B works against it). Addition climbs; concession reverses.
不…但…
不但 is one unit meaning not only; 不…但… is a negation 不 in one clause and a contrast 但 in the next. The frame here adds; that pair contrasts.
不但好吃，但是很貴。 (turning against with 但是 after 不但) → 不但好吃，還很便宜。 (second half adds, headed by 還/也)
他不但會說中文。 (不但 opened with nothing stacked on it) → 他不但會說中文，也會說日文。 (a second clause completes the stack)
不但他來，也我來。 (subject after 還/也) → 他不但來了，也帶了禮物。 (keep one subject; 還/也 leads the predicate)
English 'not only … but also' uses 'but', tempting 但是 in the second half; here the second half takes 還 or 也, and 但是 would flip the meaning to a contrast.