ばよかった

ばよかった is regret: "I should have…", "I wish I had…". It is the ば-form of a verb followed by よかった, the past of いい – word for word, "it would have been good if…". The thing you regret is something you didn't do.

For something you did and wish you hadn't, put the verb in the negative: ない becomes なければ.

The past is carried by よかった, so the verb before it stays in the ば-form. For する that is すれば.

• × 勉強しればよかった → 勉強すればよかった

ばよかった or たらよかった

たらよかった means the same and is common in conversation: もっとべんきょう勉強したらよかった. ばよかった is the form in textbooks and in writing, and polite speech adds です to either: はやく早くいえば言えばよかったです.

Usage notes

These go beyond your own regret.

ばよかったのに

With のに, the regret turns outwards. Said to someone else, it is a mild reproach: you should have, so why didn't you?

Advice in hindsight

Asked as a question, it looks back for what would have worked.