The reason________he didn't come was _____he was seriously injured in a car accident.

他 没 来 的 原 因 是 他 车 祸 重 伤 。
他 之 所 以 没 来 是 他 在 车 祸 中 受 了 重 伤 。
The reason why he didn't come was: 他 没 来 的 原 因 是

he was seriously injured in a car accident: 他 在 车 祸 中 受 了 重 伤

The reason (why he didn't come) was that--- That was the reason. 那 就 是 原 因 。 原 因 就 是 那 个 。

That was the reason why he didn't come: he was seriously injured in a car accident.

The reason___why_____he didn't come was ___that__he was seriously injured in a car accident.

the reason why 固定搭配“...的原因”

后一空that，宾语从句的引导词

The reason__why______he didn't come was __that___he was seriously injured in a car accident.