"Wen"nimmst du mit in die Oper

 I have just seen this sentence which means “who do you take to the opera with you”.
When should you use “wen” instead of “wer” when referring to who?
 

It is to do with the case. Wer is nominative and wen is accusative.
Du is the subject in this sentence and wen is the accusative object.
It is all explained at length in this video:
https://www.youtube.com/watch?v=HOPaYdxKP_A