What's the difference between "sadness", "miserable" and "melancholy"? Are these words interchangeable?

Native speaker’s answer
Rebecca
They're not interchangeable, but they are related. "Miserable" and "melancholy" are different forms of "sadness". "Miserable" is more intense or dramatic than just being sad. "Melancholy" is a state of sadness, maybe without cause, and comes from deep thought. "Melancholy" is similar to depression in its tone and expression. You can use "sadness" in different ways, but it's a simple way of saying you're "sad". Ex: Jane was sad that she couldn't go on vacation with her friends. Ex: Henry had been in a state of melancholy for the past week. I was worried about him. Ex: I was miserable working there. I'm glad I quit!