[ばform]よかった

トイレットペーパーを かえば よかった。

a. I’m glad I bought toilet rolls.
b. I should have bought toilet rolls.
c. It was good that I didn’t buy toilet rolls.
d. It will be good if I buy toilet rolls.

[ばform] is used to express a condition and is usually translated as “if” or “when.”

よかった is the past tense of the いadjective いい, which means “good.”

Combining these two, [ばform]よかった is usually used to express a regret, “I should have….” Thus, the correct answer is b.

あのとき かれと けっこんすれば よかった。
I should have married him then.
カレーに すれば よかった。
I should have chosen a curry.
こうこうの とき、もっと べんきょうすれば よかった。
I should have studied more when I was in the senior high school.
本を もってくれば よかった。
I should have brought a book.
パーティーに いけば よかった。
I should have gone to the party.
くすりを のめば よかった。
I should have taken the medicine.
もっと べんきょうしておけば よかった。(This one is hard because I used ～ておく)
I should have studied more in advance.

If you are telling somebody else that he/she should have done something, you usually add のに at the end. However, don’t use it too often as it can sound quite patronizing.

おかあさんを つれてくれば よかったのに。
You should have brought your mother with you.
あの ワンピースを かえば よかったのに。
You should have bought that dress.
エイミーさんは じょゆうに なれば よかったのに。
Amy should have become an actress.