What went through Tom's mind I can't imagine, but he did manage to keep a straight face.

汤姆心里在想什么我想像不出来，但是他确实装着一脸严肃。

We often keep a straight face or use an unsmiling expression when we're making a joke.

当我们开玩笑的时候，我们经常绷着脸或使用不笑的表情。

He kept a straight face and remained speechless.

他绷着脸不说话。

I kept a straight face as long as I could, but I finally had to crack up.

我尽量板着面孔，但最后还是控制不住失声大笑起来。

I Tried tp keep a straight face.

试着摆出一张严肃的脸。