買わなければ

if [someone] doesn't buy; unless [someone] buys

→ 買う
買わ buy + なけれ Negative conditional + Conditional (if/when)
verb · conditional Rare #31,443
if [someone] doesn't buy; unless [someone] buys
JA: 今買わなければ、売り切れてしまう。
EN: If we don't buy it now, it will sell out.
Conjugation of 買う
Conditional
買ったら買わなければ

私は明日忘れずにその本を買わなければいけない

I must remember to buy that book tomorrow.

家を買わなければお父さんが君を他の人と結婚させるんだよ

If you don't buy a house, your father will make you marry someone else.

— バジュランギおじさんと、小さな迷子 (2015)

こんなものを買わなければ良かった

I wish I hadn't bought this.

あんな役に立たないものなど買わなければ良かったのに

I wish I hadn't bought such a useless thing.