How to tell the difference between “再 zài” and“又yòu”

"再zài” and “又yòu” both mean repetition and continuity of an action,

but "再zài”  refers to something unrealized;“又yòu”refers to something realized.

For example:

请再说一遍。Qǐng zài shuō yí biàn.  Please say it again.

他又来了。Tā yòu lái le.  He came again.