He was breathing through his open mouth.

他张着嘴呼吸。

《牛津词典》

Perhaps, then, an open mouth is more a sign of some impending change of state rather than a statement of tiredness itself; awake to asleep or vice versa.

如此说来，也许张嘴打哈欠只是一种即将改变状态的信号，而不是疲倦本身的表现；由醒到睡，反之亦然。

youdao

Fire came from its open mouth.

火从它那张着的嘴巴里喷了出来。

youdao

The baby will open its mouth twice, the second time with a nice wide mouth, and lean in to grasp the breast.

VOA: special.2010.08.04

As the fierce winds charged her belly, Her body was distended and her mouth was wide open.

这时邪风充满了她的肚子，她的身体膨胀起来，她的嘴长得很大。

耶鲁公开课 - 旧约导论课程节选

You have to stop eating with your mouth open.

你不能再边说话边吃饭了。

have to 实战 - SpeakingMax英语口语达人

The very first thing we see, for example, is an open mouth, belonging to a man under interrogation.

NEWYORKER: Better Times

After the dead bear arrives for mounting, taxidermists hit customers with a key question: open mouth or closed mouth.

WSJ: After Long Hibernation, Taxidermists Make a Killing During Bear Season

"The human head had a light brown colour, open mouth and partially closed eyes, " said the scientists, led by forensic pathologist Philippe Charlier.

BBC: Tests show head of France's King Henri IV 'genuine'