まだ読んでいない本が十冊もあるから、しばらく本は（ ）と決めていたのに、面白そうな本を見つけて、つい買ってしまった。
Fill the bracket with the option provided below:
a) 買わないでいたんだ
b) 買わずにおこう
Which is proper answer? I think both are acceptable, (a) comes from pattern ないでいる, which is the same with pattern ずにおく, but the correct answer is (b). But I don't know why, can anyone help me explain it?
The option a) is incorrect for two reasons:
Japanese is based on relative tense, so you cannot use た with 決める. For example, "I decided that I would go" is 行くと決めた or 行こうと決めた, not 行ったと決めた. The action of "going" always happens after the action of "deciding" no matter when this sentence is about.
What's that ん? Is is supposed to be an explanatory-の? The part before a quotative-と must be a "mini-sentence", and you cannot end a sentence with ん.
In this context, you can say the following:
