不客气

-- Thank you. -- You are welcome. -- 谢谢. -- 不客气/不用谢/没关系.

不客气/别客气/不用谢

不bu(second tone)客ke(4th tone)气qi(4th tone) 不is second tone here, however, it is the 4th tone if you read it for single, or you want to just say no, then this不， is the 4th tone

一般情况下用在别人和你说“谢谢”的时候。你可以回答“不客气”“不用谢”