It was a pretty serious accident.

这是一次相当严重的事故。

This is the most serious accident this year .

这是今年最严重的事故。

His carelessness resulted in a serious accident.

他的粗枝大叶导致了一场重大事故。

A serious accident happened on the highway yesterday.

昨天高速公路上发生了一起严重的车祸。

Your prompt action prevented a serious accident.

你即时的行动防止了一次严重的事故。