他突然严肃起来。

He suddenly sobered.

《牛津词典》

他脸上的神情突然严肃起来，声音也压低了。

His face suddenly turned solemn, his voice confidential.

youdao

我难以置信地盯着那名球员。他打破了沉默。“你不会发推说这件事的，对吧？”他说，突然严肃了起来。

I gave the player an incredulous stare. He broke the silence. "you aren't going to tweet about that are you?" he said, suddenly serious.

youdao