At a glance
たが最後 means "once something happens, there is no going back". A result follows that can't be stopped, and it is usually bad or out of control.
Verb (た-form) + が最後、[result that can't be stopped]
彼はお酒を飲み始めたが最後、朝まで飲み続ける。 Once he starts drinking, he keeps drinking until morning.
How to form it
Put the verb in its plain past affirmative (た-form) and add が最後.
| Verb group | Dictionary form | + が最後 |
|---|---|---|
| う-verb | 飲む | 飲んだが最後 |
| う-verb | 言う | 言ったが最後 |
| る-verb | 見る | 見たが最後 |
| る-verb | 始める | 始めたが最後 |
| する | 約束する | 約束したが最後 |
| 来る | 来る | 来たが最後 |
The second clause states the result, and it is often followed by a comma.