条件・譲歩N1
〜が最後、〜たら最後
once... then... / the moment... (something bad follows)
Structure
V-た形 + が最後／たら最後
Explanation
Once the first action happens, an irreversible (usually bad) result follows. The main clause is usually negative.
Examples
ゲームを始めたが最後、止まらなくなってしまう。
Once I start a game, I can't stop.
あの店に入ったら最後、財布が軽くなる。
Once you step into that shop, your wallet gets lighter.
Note
The main clause is usually irreversible or bad. Unlike たとたん, it stresses that the result can't be undone.