Expressing "Should have" with ～ばよかった

The phrase ～ばよかった can be used to express "I wish I had done X" or "I should have done X." It's used to describe an action that you regrettably did not take.

The ば comes from the verb that precedes the phrase. To get this form of the verb, regardless of whether it's ichidan or godan, take the dictionary form, change the final う sound to the え equivalent, and add ば.

Positive Forms:
食べる  ->  食べれば
行く       ->  行けば
死ぬ     ->  死ねば
待つ     ->  待てば
走る     ->  走れば
感じる  ->  感じれば

To get the negative ば form, take the verb in the plain negative form, drop the final い and add ければ.

Negative Forms:
食べる  ->  食べなければ
行く       ->  行かなければ
死ぬ     ->  死ななければ
待つ     ->  待たなければ
走る     ->  走らなければ
感じる  ->  感じなければ

学校の人にお金を施せばよかったですよ。
I wish I had donated money to the people at the school.

あのかびたチーズを食べなければよかったでしょ。
I probably shouldn't have eaten that moldy cheese.

日本に来る前に、新しいフォーンを買えばよかったです。
I should've bought a new phone before coming to Japan.

あの男を殺さなければよかったけど、大嫌いだったんだよ。
I wish I hadn't killed that man, but I really hated him.