What is the critical functional difference between '-다면' and '-았/었더라면' conditional clauses, specifically in the context of expressing counterfactual situations?

The critical functional difference lies in the degree of hypotheticality and the implied possibility of the condition being true. '-다면' expresses a hypothetical condition that is still potentially possible, or at least open to some possibility, even if unlikely. It sets up a scenario that might occur in the future or under certain circumstances. The speaker is considering a situation that isn't necessarily contrary to fact. For example, '내일 비가 온다면 집에 있겠다' (If it rains tomorrow, I will stay home) presents a future possibility. In contrast, '-았/었더라면' expresses a counterfactual condition – a condition that is contrary to past fact; the condition *did nothappen, and the speaker is reflecting on what *would havehappened if the condition had been true. This implies regret, unrealized possibilities, or a situation that cannot be changed. For example, '돈이 있었더라면 그걸 샀을 텐데' (If I had had money, I would have bought that) indicates that the speaker did not have money, and consequently, could not buy the item. Therefore, '-다면' presents a possible (though perhaps improbable) future scenario, while '-았/었더라면' expresses a past impossibility and reflection on an alternative reality.