Yeah, and it's too bad they don't have a "slow button" yet. Just keep at it and you'll eventually get it. I personally think Vietnamese is harder than French and Spanish. I take such a long time doing exercises in the Vietnamese course. Whereas with French and Spanish, I just whip past through them. Keep going!
In general, khi nào is placed in the beginning of the sentence when asking about something that happens in the present/future and is placed at the end of the sentence when asking about something that happened in the past.
Khi nào bạn học tiếng Viẹt?
When do you study Vietnamese?
Bạn học tiếng Việt khi nào?
When did you study Vietnamese?
For more in depth grammar, I recommend "Elementary Vietnamese" by Binh Nhu Ngo.