这个句子冠词写错了地方，还漏掉了 one：

Used with a singular countable nouns to refer to one of number of things or people, when it doesn't matter which.

应为：Used with singular countable nouns to refer to one of a number of things or people, when it doesn't matter which one.

1. 这是解释 any 的用法。when 引导时间从句，这很好理解。可以理解为“在此情况下，......”

2. matter 是不及物动词，which one 不是宾语，而是省略了成分 it refers to（it 指any），完整的表达是：

 it doesn't matter which one it refers to.

3. 上句，it 是形式主语，which one it refers to 是真正主语外置。意思是：它指的是哪一个无关紧要。或：它指哪个都行。

关于第二个释义，也是如此。