future or present or past tense in this example sentence? Hello. I am not sure which one to pick in the sentence below. I found out that she would not be available until next month, when she (will come back / comes back / would come back / came back) from business trip. I would appreciate if someone could tell me which one is correct (and hopefully explanation too). Thank you!

Hej! Konnichiwa. I am looking for a Danish friend who can do language/culture exchange with me. I would like to study Danish and would like to know about Denmark (culture, music, anything about Denmark). I am Japanese, so I can teach you Japanese and I can tell you about Japan (our culture, popular music, etc.) in return. I have just started studying Danish, so hopefully someone who can be patient with me. (undskyld!) Please message me if you are interested. Yoroshiku onegaishimasu. :)