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知ってたら

if (someone) knew, if (someone) had known

→ 知る
知っ to know + て Te-form (conjunctive form) + た Past tense + ら Conditional (if/when)
verb · informal, past tense Rare #32,690
if (someone) knew, if (someone) had known Colloquial contraction of 知っていたら.
JA: それを知ってたら、先に連絡したのに。
EN: If I had known that, I would have contacted you earlier.
Conjugation of 知る
Conditional past
知ってたら

本当のこと知ってたら言うって。

If I knew the truth, I’d tell you.

知ってたらよかったんだけど。

I wish I had known.

何を知ってたらいいの？

What do I need to know?

— 動くな、死ね、甦れ！ (1990)

知ってたら来なかった

If I had known, I wouldn’t have come.

— ファニーゲーム (1997)

知ってたら何なの

So what if I knew?

— Carandiru (2003)

本当のことを知ってたら、あなたにお話しましたよ。

If I had known the truth, I would have told you.