たら最後 / たが最後|たらさいご / たがさいご

Once X happens, a usually undesirable result follows that is hard or impossible to avoid. The first action is presented as a trigger after which the situation slips out of one's control. The result need not be permanent.

Used to warn or predict that doing one thing will unavoidably bring about a negative or troublesome situation. The main clause often ends in a prediction or judgment such as だろう or よ. たら最後 sounds more conversational, while たが最後 has a slightly more literary feel. Unlike 'as soon as' patterns such as なり or が早いか, the focus is on the unavoidable consequence rather than on timing.

• 1.
  が最後N1「が最後」 is exactly the same as 「たが最後」, and both attach to the verb ta-form. When used for a negative result, 「が最後」 and 「たら最後」 are interchangeable, but 「が最後」 is more literary and stronger in tone, and its second clause can also express a delightful result that leaves you unable to stop.
• 2.
  そばからN1「そばから」 stresses immediate succession, repetition, and effort coming to nothing, and its second clause states what actually happens; 「たら最後」 stresses that once something is done it cannot be undone, and its second clause is usually a conjecture or firm judgment. 「そばから」 can also attach to the dictionary form, so the two cannot be interchanged.
• 3.
  なりN1「なり」 attaches to the dictionary form and signals that an unexpected action immediately follows the first, with the second clause usually a fact that has already happened; 「たら最後」 attaches to the ta-form and focuses on irreversible negative consequences, with the second clause usually a conjecture or judgment. The two cannot be interchanged.
• 4.
  が早いかN1「が早いか」 stresses that the two actions occur back to back; its second clause must be a fact that has already happened, it can attach to either the dictionary form or the ta-form, and it is more literary. 「たら最後」 focuses on irreversible negative results, its second clause is usually a conjecture, and it attaches only to the ta-form, so the two cannot be interchanged.
• 5.
  ばそれまでだN1「ばそれまでだ」 is placed at the end of the sentence and focuses on prior efforts or achievements being wasted, and it can also follow adjectives or nouns. 「たら最後」 attaches only to the verb ta-form and stresses the irreversible negative consequences once it is triggered, so the two generally cannot be interchanged.

が最後 is essentially the same construction, used after the た-form as たが最後. なり and が早いか mean 'as soon as' and focus on immediate timing rather than an unavoidable consequence, while そばから describes an action repeatedly undone right after it happens, and ばそれまでだ means 'if X happens, that's the end of it,' declaring everything wasted rather than predicting a further consequence.