不是…就是…grammar point · tier 2 · 不是…就是… either/or (one of two, no third)
Exhaustive alternation: rule out the first option and the second is what stands; it is always one of the two.
字源 FORM what the parts do
不 rules the first option out, the bird that will not come down. 就 settles on the one that is left, the step already taken. 是 stands in both halves as the copula, fixing each option as the case. Together they fence in two choices with nothing between.
故事 STORY a scene to remember it by
A switch with two positions and no middle; flick it off the first and it drops onto the second.
自撰字源記憶法
框 · Frame
不是 [option A]，就是 [option B]
觸 · Trigger
You want to say it is always one of exactly two things, never a third.
序 · The move
1name the topic the two options describe
2不是 + the first option you rule outis this a closed set of two, with no third left open?
3就是 + the second option that then standsif the first is false, is the second forced?
例 · Examples
1他週末weekend不是打籃球play basketball，就是去慢跑jog。
On weekends he's either playing basketball or out jogging.
或是 / 還是 offer open alternatives you can pick among; 不是…就是… seals the set at two and forces one of them. Choice versus exhaustion.
不但…而且…
不但…而且… stacks both halves (both are true); 不是…就是… splits them (one or the other, not both at once).
dropping 就: 他不是打籃球，去慢跑 → 他不是打籃球，就是去慢跑
wedging in a third: 不是A，就是B，就是C → 不是A，就是B (closed at two)
reading 不是 as plain negation: 他不是去慢跑 'he isn't jogging' ≠ the frame; the frame needs the 就是 half to close it
English 'either … or …' is symmetric and front-loaded with 'either'; learners reach for a single conjunction and skip the negated first half, or use 或是 where the set must be sealed at two.