"I sing when I am running."
Translation:Tôi hát khi tôi đang chạy.
Would I not use "khi nào" here (i.e., "Tôi hát khi nào tôi đang chạy.")? If not, I'm not sure I understand where to use "khi" vs. "khi nào".
khi nào = when, whenever. When you talk about duration, especially when you can replace "when" with "while", just use khi.