（要）不然grammar point · tier 1 · （要）不然 otherwise / or else (the bad branch if you don't)
Names the unwanted consequence that follows if the first clause is not acted on: do X, or else Y happens.
字源 FORM what the parts do
不 is the borrowed negation, the bird that will not come down. 然 carries its loaned sense 'so, this way' — the blaze grammar took for thus. 不然 reads 'not this way': set the first clause aside and the second branch is what is left. 要 fronts it as the thing to head off.
故事 STORY a scene to remember it by
One road forks. Stay on the first and all is fine; step off it and 不然 swings you onto the worse road waiting alongside.
字源記憶法
框 · Frame
[clause: do X / a warning]，（要）不然 [the bad consequence Y]
觸 · Trigger
You want to spell out the bad thing that happens if the first clause is not honored.
序 · The move
1state the first clause: the action to take or the warning given
2（要）不然 at the head of the second clausedoes the second clause name the unwanted outcome of NOT doing the first?
3the bad consequence Y that the first clause was meant to avoidwould the consequence actually follow if the first clause is dropped?
如果…就… lays a condition and then its consequence (if X, then Y). （要）不然 takes the first clause as given and names what happens on the NEGATIVE branch — the consequence of NOT doing it. Stated condition versus the unstated 'or else'.
否則 is the same move in a written, formal register; （要）不然 is the spoken everyday form. Same junction, different register.
treating it as plain 'and then': 我們動作快，然後船要開走了 → 我們動作快，不然船要開走了 (the second clause is the avoided bad outcome, not the next event)
putting it after the consequence: 船要開走了不然動作快 → 動作快，不然船要開走了 (不然 heads the bad branch, not the good action)
stacking it onto a stated 如果 condition: 如果不快，不然船開走 → 如果不快，船就開走了 (use one frame; 不然 already carries the negative if)
English 'or else / otherwise' floats freely and can trail at the end of a sentence; learners drop it after the consequence or pair it with a fully spelled-out 'if not' clause, doubling the conditional.