别客气。

Use it immediately after someone says 谢谢. It is short, punchy, and effective. You can say it with a smile and a slight nod. If you want to sound extra warm, say it twice: 别客气，别客气. This doubling is very common in spoken Chinese. It shows you really mean it. It is like saying 'No worries at all!'

Use it in almost any daily situation. Use it when you hold a door open. Use it when you lend a pen to a classmate. It works perfectly when you help a colleague with a small task. It is great for texting after a friend thanks you for a gift. It is the 'Swiss Army knife' of polite responses. You will hear it dozens of times a day in China.

Avoid it in extremely formal ceremonies with high-ranking officials. In those cases, use 不客气. Don't use it if someone is apologizing to you. It only responds to thanks, not 'I'm sorry.' If a stranger is being aggressive, this phrase won't help you. Also, don't use it if you are actually angry at the person. It sounds too friendly for a heated argument!

Chinese culture values the 'Guest-Host' dynamic deeply. Being 'polite' (客气) can sometimes imply a distance between people. By saying 别客气, you are breaking down that wall. You are saying, 'We are close enough that you don't need to thank me.' It reflects the importance of 'Guanxi' or social connections. It turns a transaction into a relationship.

You will often hear 不客气, which is slightly more standard. Friends might just say 没事 (It's nothing). In the south, you might hear 不用谢 (No need for thanks). If someone is being too shy, you can say 跟我还客气什么?. This means 'Why are you being polite with me?'. It is a very endearing way to show friendship.