If only I could remember where it was that I saw someone putting away the box with those candles in,

remember的宾语从句用了强调句型，强调where这个地点。如果不强调，改用普通句式则为：

If only I could remember where I saw someone putting away the box with those candles in,

这样你就可以看出where引导宾语从句，并在从句中作putting away the box的地点状语（为现在分词短语中的成分）。with those candles in为with复合结构作box的定语。

整个句子为虚拟条件句，假设的条件与事实相反。事实我不记得装蜡烛的盒子放哪里了，所以我也无法完成装饰。