1. Im sad.

2. Im unhappy.

3. Im in bad/dark mood.

△假如说又生气了，还可以用到mood的有关短语：

be in one of your moods，表示又心情不好；又生气。

Tims in one of his moods so Im keeping out of his way.

蒂姆又闹情绪了，所以我就离他远点儿。

4. Im in low spirits.

△low spirits就是无精打采；沮丧，忧郁。假如说lift sbs spirits，就是提升（某人）的兴致。

Nothing - not even the prospect1 of dinner - could lift his spirits.

什么都提不起他的兴致，甚至连吃饭也不例外。

5. I feel so down.

△这里的down是形容词，意思是不开心的；提不起精神的。

Ive been a little bit down this week.

这周我有点情绪低落。

down还可以做名词哦，譬如have a down on sb就表示不喜欢，对有偏见。

Why do you have a down on him? I think he seems really nice.

你为何不喜欢他？我感觉他很好。