I'll never forget the days------I stay with you.

I'll never forget the days when I stay with you.
因为I stay with you是the days的定语.
而when在I stay with you中又作时间状语成分.
stay作及物动词和不及物动词都可以.
至于stay是及物动词还是不及物动词的问题,则要根据题目具体而言.本题是不及物动词.