"I feel like this is the rule for if we us a/the/ or which way we word a sentence" ?

I agree that some translations are not always the best.

However, that's why the course is still in beta. As beta user, you can suggest alternative translations or flag inaccurate existing ones.

Duolingo's business model is based on volunteers and crowdsourcing rather than hiring professional translators.

There exist other paid resources if that's what you're looking for.

As for this specific case, I would translate it as "it depends" or "it is different each time", as they did here.

Because 그때 그때 specifically means each time