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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