〜ばよかった
I should have, I wish I had
Verb ば-form + よかった · なければよかった
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ばよかった is regret: literally, "if I'd done it, it would have been good". 傘を持ってくればよかった, "I should have brought an umbrella".
For something you wish you hadn't done, use the negative ば-form: 食べなければよかった, "I shouldn't have eaten it".
Add のに for a wistful or reproachful ending: もっと早く言えばよかったのに, "you should have said so sooner". Aimed at someone else, it's a gentle complaint.
In casual speech the たら version is just as common: 聞いたらよかった. The present form, ばいい, looks forward instead and gives advice.
Examples
雨だ。傘を持ってくればよかった。
It's raining. I should have brought an umbrella.
もっと早く起きればよかった。
I should have got up earlier.
あんなに食べなければよかった。
I shouldn't have eaten so much.
先生に聞けばよかった。
I should have asked the teacher.
もっと早く言えばよかったのに。
You should have said so sooner.