板起脸

例句：

1. 1. He always speaks with a stern face when he is angry.
   他生气时总是板着脸说话。
2. 2. She tried to hide her disappointment by maintaining a serious expression.
   她试图通过保持严肃的表情来掩饰她的失望。
3. 3. The teacher was stern when she gave the students their homework.
   老师布置作业时表情严肃。