当我开别人玩笑时，很难保持严肃（忍住不笑）。

It's hard to ___keep ___a ___straight ___face when I'm playing a joke on someone.

在那场严肃的会议上，他那个有趣的评论差点让我没绷住脸（差点笑出来）。

During the serious meeting, his funny comment almost made me fail to ___keep ___a ___straight ___face.

情况太荒谬了，她在整个解释过程中都努力保持着严肃的表情（忍着不笑）。

The situation was so absurd that she struggled to ___keep ___a ___straight ___face throughout the explanation.