Explanation
Emphasizes 'once something happens, there is no going back'; 'once', 'as soon as', etc.
Formation
Verb-casual, past (た form) + が最後
Detailed explanation
The 〜たが最後 grammar point indicates that once an action is taken or event occurs, some (often irreversible) consequence follows. In English, it can be translated as 'once', 'as soon as', or 'the moment that...'.
Examples
Kare ga ichido okotta ga saigo, dare mo kare o tomeru koto wa dekimasen.
Once he gets angry, no one can stop him.
Kono gēmu o hajimeta ga saigo, hitobanjū yari tsuzuketa.
As soon as I started this game, I played it all night.
Kanojo ga heya o deta ga saigo, nido to modotte konakatta.
The moment she left the room, she never came back.
Watashi ga sono hashi o watatta ga saigo, sore ga saigo no wakare datta.
Once I crossed that bridge, it was our final goodbye.
Practice
Grammar content sourced from hanabira.org.