1. 的确，in which可以等于when，也可以等于where，但不会等于why.

关键是看定语从句中缺少时间状语还是地点状语。

2. 例如：

1）I'll never forget these days in/during which I stayed with you.

= I'll never forget these days when I stayed with you.

我永远不会忘记我和你待在一起的日子。

其中：in/during which I stayed with you = when I stayed with you

= I stayed with you in/during which = I stayed with you in/during these days

2）This is the city in which I stayed last year.

= This is the city where I stayed last year.

这就是那个去年我待的地方。

其中：where I stayed last year = in which I stayed last year

= I stayed in which last year = I stayed in the city last year.

3. 另外，有的in which找不到合适的关系副词代替。

如：I dislike the way in which you treat me.

我不喜欢你对待我的方式。

其中：in which you treat me = you treat me in which

= you treat me in the way

4. for which = why

如：Can you tell me the reason why you were late?

= Can you tell me the reason for which you were late?

你能告诉我你迟到的原因吗？

其中：why you were late = for which you were late

= you were late for which = you were late for the reason