The ば-forms of a verb represents hypothetical situation or activity.
For ば-forms of the affirmative, drop the final vowel "u" and add "eba".
ex)
たべる(taberu)→たべれば(tabereba)
いく(iku)→いけば(ikeba)
くる(kuru)→くれば(kureba)
For ば-forms of the negative, drop the final い and add ければ.
ex)
たべない→たべなければ
いかない→いかなければ
こない→こなければ
ばよかったです means "I wish I had done something" or "I should have done something".
なければよかったです means "I wish I had not done something" or "I should not have done something".
<Example Sentences>
あさごはん を たべればよかったです。
I should have eaten breakfast.
きのう パーティ に いけばよかったです。
I should have been to the party yesterday.
よなか に ラーメン を たべなければよかったです。
I should not have eaten a ramen noodle in the middle of the night.
きのう パーティ に いかなければよかったです。
I should not have been to the party yesterday.
References:
Verb Conjugation
https://anchor.fm/yuya-sato/episodes/Verb-Conjugation-emv716
Te-forms
https://anchor.fm/yuya-sato/episodes/Te-forms-enfaqj
Volitional Form
https://anchor.fm/yuya-sato/episodes/Volitional-Form-epmave
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