French language Q&A Forum

I have a question which comes from this exercise but is not directly related to the subject matter perhaps but is a listening issue that I have come across before and there must be a rule that I don't know about or that I do and just don't recognise the context: 

In reflexive verb video, which is in the last set of "your practice" offered after you complete the last written section; at 7'40" in the video, the lady says: "Et puis, qu'est qu' on fait quant on utilise l'imperatif..." but instead of saying "quand" she pronounces the "t" as if it were "quante" ...So I wonder if you could tell me what the rule is for pronouncing "quand" with a "t" at the end, since quant, is of course a different word...

I hope that makes sense. I look forward to hearing from you. 

Best regards,

Michael

I feel like sometimes I've seen these used in an interchangeable manner (devoir and avoir à). What I told my students was that "devoir" also means "must", so "avoir à" needs to be used in situations where "must" doesn't fit (i.e. "You don't have to go yet." "Tu n'às pas encore à partir."). Is this correct? In what other situations do you need to use one over the other?