Why is 買わないでいたんだと決めていた incorrect?


> 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](https://japanese.stackexchange.com/q/25376/5010), 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:

  * しばらく本は買わずにいようと決めていたのに
  * しばらく本は買わないでいようと決めていたのに
  * しばらく本は買わずにおこうと決めていたのに
  * しばらく本は買わないでおこうと決めていたのに
  * しばらく本は買わないん **だ** と決めていたのに (notice だ after ん)
  * しばらく本は買わないと決めていたのに

