ばよかった

1. 1Expresses regret about something the speaker did not do.Ex. 1
2. 2Expresses regret about preparation the speaker did not make.Ex. 2
3. 3Directed at another person, expresses a hindsight comment or mild reproach.

Neutral in register, ばよかった looks back on an action not taken; it often appears with もっと or あの時, and ておけばよかった expresses regret about a lack of preparation. Unlike といい / たらいい, it is not used to give advice about what to do next.

• ものをN1「ものを」is a literary adversative conjunction often placed after 「ばよかった」 to introduce an outcome contrary to what was hoped, carrying a stronger tone of reproach; its preceding clause can also state an established fact, and it can end a sentence on its own. Since 「ばよかった」 is stylistically neutral and can express regret by itself, the two should not be directly swapped.
• のにN4「のに」attaches to plain forms and marks an adversative, pointing out facts contrary to expectations, while 「ばよかった」 attaches to the conditional ba-form and expresses regret by imagining how things could have gone differently. They cannot replace each other, but they frequently combine as 「〜ばよかったのに」, adding a nuance of sorrow or blame.
• てよかったN4「てよかった」attaches to the te-form and expresses relief or joy about something that has actually happened, including happiness on someone else's behalf, whereas 「ばよかった」 laments that reality fell short of one's wishes—their emotional directions are opposite, so they cannot be interchanged. For example, 「行かなくてよかった」 means being glad you did not go.
• といい / たらいいN4「といい / たらいい」focuses on things not yet realized, expressing wishes, offering suggestions, or asking how to do something, while 「ばよかった」only looks back on a settled past with regret and cannot give advice. Both can take 「のに」, but their time orientation differs.
• ものならN2While 「ばよかった」expresses regret over a past whose outcome was already decided, 「ものなら」with a potential form hypothesizes hard-to-realize situations followed by a wish or a challenge, and with a volitional form it serves as a warning that doing so would bring serious consequences. The two cannot substitute for each other.

てよかった expresses gladness about what happened, while といい / たらいい can express advice or wishes about the present or future; ばよかった looks back with regret. のに can add frustration to ばよかった, whereas ものを expresses similar regret or reproach in a literary style; ものなら introduces a hypothetical condition rather than hindsight regret.