それまでだIf… then it is all in vain, It's done, It is useless, No point, It is a waste, That's it, It is all gone,

Both それまでだ and たまでだ use までだ to mark a limit, but the type of limit is different. それまでだ expresses a harsh evaluation meaning “that’s all there is” or “that’s the end,” often used to judge that nothing more can be done if a condition is lost, as in 信頼を失ったらそれまでだ (‘If you lose trust, that’s the end.’). By contrast, たまでだ uses the past tense to downplay an action and means “I only did (A),” emphasizing that the action was small or not worth blaming, as in 俺はただ自分の意見を言ったまでだ (‘I only expressed my opinion.’) or 困っていたから助けたまでだ (‘I just helped because she looked troubled.’).

Both involve a limit, but the type of limit is different. それまでだ is an evaluative expression meaning “that’s the end,” used to judge that nothing more can be done if a crucial condition is lost, as in 信頼を失ったらそれまでだ (‘If you lose trust, that’s the end.’). By contrast, Vるまでだ (and までのことだ) expresses resignation, meaning “one can only but (B),” often used with なら to show that (B) is the only remaining option in a negative situation, as in 誰にも教えてもらえないなら、ネットで調べてみるまでだ (‘If no one will teach me, I’ll just have to look it up online.’).