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。

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。

She said it with a straight face.

她板着脸说了这件事。

Dont keep a straight face.

别板着脸吓唬我。