我要装出一副严肃的面孔很难。

I found it hard to keep a straight face.

youdao

差不多紧靠屋角，有个老兵坐在军大衣上面，露出一副骷髅般瘦黄的、严肃的面孔、没有剃过的苍白的髯须，他目不转睛地望着罗斯托夫。

Almost in the corner there was, sitting on a military overcoat, an old soldier with a stern yellow face, thin as a skeleton's, and an unshaved grey beard. He was looking persistently at Rostov.

youdao