Im a bit down in the mouth today.

我今天有点闷闷不乐的。

I'm a bit down in the mouth today.

今天我胃口不是很好。

Because he looks down in the mouth.

因为他总是往嘴里看。

You look down in the mouth. What's the matter?

你看起来很沮丧，出什么事了？

Since her dog died, Susan has been down in the mouth.

苏珊的狗死了后，她一直愁眉苦脸的，闷闷不乐的。