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ただろう

would have, probably did

→ だ
た Past tense + だろう Conjectural
auxiliary · informal, past tense Rare #43,358
would have, probably did Often used after a verb in the colloquial contraction ～てた, expressing a guess or a hypothetical past action.
JA: 彼も来てただろう。
EN: He probably came too.
Conjugation of だ
Subjunctive past
だったろただろう

どれだけ清潔な胃になってただろうね

I wonder how clean your stomach would have been.

— 親切なクムジャさん (2005)

私はそのことを別の言い方で言ってただろう。

I would have said it another way.

学校の床を掃除していたの見てただろう？

You saw me cleaning the school floors, didn’t you?

— Article 15 (2019)

これから起きる事をすべて知っていたら－結果は違ってただろう

If I had known everything that was going to happen, the outcome would have been different.

— ピエロがお前を嘲笑う (2014)