Formally, “hubiera”/“hubiese” is only used within subclauses, i.e «si lo hubiera visto, habría hecho algo» etc. They never stand as the only verb and don’t appear in simple clauses. You can’t say “hubiera hecho algo”, it’s “habría hecho algo”. Here, the subjunctive (hubiera) is doing the same job as “were” in english, and the conditional (habría) is equivalent to “would”: «if I were luckier, it would have worked out». This is the case with subjunctives in general, they mostly only ever appear in subclauses other than very specific exceptions (such as negative imperative or vestigial expressions like «Dios quiera que…»). The trick is learning which subclauses use indicative and which use subjunctive :)

Informally, though, natives will tell you that it doesn’t matter because it truly doesn’t. The formula “if (subjunctive) then (subjunctive)” is understood by everybody with the same meaning as “if (subjunctive) then (conditional)”, and you can even use it in formal settings such as when talking to your boss, at least in Spain.

Source: Am native, from Spain. Good luck with the language learning!