If it had …, would have: nếu (như) … thì … đã

Vietnamese has no special verb form for "if I had …, I would have …". It uses nếu as usual, and puts đã before the verb in the result, often with rồi at the end: Nếu em biết thì em đã giúp rồi. If I'd known, I would have helped. The situation tells the listener that you didn't know and didn't help.

A time word such as hôm qua or hồi đó (back then) makes it clear you mean the past: Nếu hôm qua không mưa thì mình đã đi biển rồi. If it hadn't rained yesterday, we'd have gone to the beach. Thì before the result is usual here. Add chắc for "would probably have": Nếu học nhiều hơn thì chắc em đã đậu rồi.

Nếu như is a slightly fuller nếu, and it suits this kind of sentence: Nếu như anh nói sớm hơn thì em đã không lo.

You know nếu … thì for real possibilities. When you only want to say you regret something, phải chi or giá mà (if only) is shorter: Phải chi em biết sớm hơn. If only I'd known sooner.