To feel sad in English, you can say "to feel down" or "to feel blue." These phrases are often used to describe a general feeling of sadness or melancholy. The origins of these phrases are not clear, but they may come from the idea of feeling "low" or "down" in spirits. For example, you might say:

"I've been feeling down lately because I lost my job."

"I know you're feeling blue, but things will get better."

Here are some possible translations of these sentences into Chinese:

"我最近感觉难过，因为我失去了工作。"

"我知道你感觉难过，但事情会变得更好的。"