Re: Who to call "Sensei"?
Their is no such thing as a simple question in Aikido!
We call 4th dan and above sensei on the mat. We would usually call who ever is teaching the class sensei. We address our very senior teachers sensei on and off the mat. Not only as a sign of respect but as a reminder of there responsibly to us!
Our country highly values egalitarianism which can led to a casualness around titles...so it takes a bit of getting used to.