`伤感` is a sentimental, wistful, or nostalgic sadness. It's often triggered by something beautiful, poetic, or memorable (e.g., a sunset, an old photo, a song). The feeling is often quiet and reflective.
`难过 (nánguò)` is the general, all-purpose word for “sad,” “upset,” or “feeling bad.” It's a direct emotional response to a negative event. It literally means “hard to get through.” You use it when you fail an exam, have a fight with someone, or hear bad news.
Incorrect Usage: 你考试没及格,一定很伤感吧? (Nǐ kǎoshì méi jígé, yīdìng hěn shānggǎn ba?) - “You must feel very sentimental since you failed the exam, right?”
Why it's wrong: Failing an exam causes direct upset and disappointment, not poetic melancholy.
Correct Usage: 你考试没及格,一定很难过吧? (Nǐ kǎoshì méi jígé, yīdìng hěn nánguò ba?)