我试图讲述这个严肃的故事，但我没法板着脸。

I tried to tell the serious story, but I couldn't ___keep ___a ___straight ___face.

在正式仪式上，保持严肃至关重要。

During the formal ceremony, it was crucial to ___keep ___a ___straight ___face.

他的笑话太荒谬了，房间里没有一个人能忍住不笑。

His joke was so absurd that no one in the room could ___keep ___a ___straight ___face.