是为什么

why is that?

→ 是
phrase Uncommon #11,179
why is that? Used to ask for the reason something happened or is the case. In sentences like 这就是为什么, it means “that's why.”
CH: 他突然离开了,这是为什么?
EN: He suddenly left. Why is that?

我不知道是为什么

I don't know why.

— Suzhou River (2000)

不知为什么他十分生我的气也许你们能告诉我是为什么

I don’t know why he’s so angry with me. Maybe you can tell me why.

— 虎胆龙威 (1988)

我想知道的是为什么原因是什么

What I want to know is why.

— 地球上的星星 (2007)

你觉得是为什么呢

Why do you think that is?

— 坠落的审判 (2023)

但现在还不知道是为什么

But we still don't know why.

— 崖上的波妞 (2008)

你们那样围着,大声笑,是为什么?

Why are you all gathered around like that, laughing so loudly?

— 穆那大哥做医生 (2003)

你知不知道你一直考不上是为什么?

Do you know why you keep failing the exams?

— A Sun (2019)

你又是为什么?

Why are you doing it again?

— 搏击俱乐部 (1999)

你知道那是为什么

Do you know why?

— 风吹麦浪 (2006)