✓Say it to acknowledge effort, such as after someone carries things, finishes a report, or works a long shift. It can also express care while someone is working hard.
✓A modest reply is 不辛苦 (bù xīnkǔ), “It was no trouble.” You do not have to deny feeling tired; 谢谢 (xièxie), “Thank you,” can simply acknowledge their appreciation.
✓If the other person also worked hard, 你也辛苦了 (nǐ yě xīnkǔ le), “You worked hard too,” shares the appreciation. Use it when that is true of the situation.
✓For thanks after a small favor, 不客气 (bù kèqi), “You’re welcome,” may also fit. No single reply is required every time.
✓It recognizes effort; it does not automatically replace 谢谢 for receiving a gift or routine information. At work, the relationship and delivery affect whether it sounds collegial, caring, or evaluative.