Q:

遲 vs. 晚 in Mandarin: when to use which one?

In Cantonese, you can use 遲 to indicate someone is late (e.g., not punctual) or the time is late (e.g., past midnight so all restaurants are closed).
However, it seems like Mandarin speakers use 晚 in place of 遲 most of the time.
What are the Mandarin guidelines for using 遲 vs. 晚? Should you just always use 晚?

A:

晚 and 遲 are interchangeable when it is used as an adverb for "late"
For example, 晚到= 遲到; 來晚了= 來遲了
But 遲 have the additional meaning of "delay"
Example: 

You can say 遲遲才回覆 (reply after long delay/ reply very late) but you can't say  晚晚才回覆
You can say 遲則有變 (situations change when you delay your action/ situations change when you are late to act) but you can't say  晚則有變 

