This is teaching, Yuya. Sometimes you need to figure out what you did wrong yourself so that you don't do it again (or so Strange thinks).

"Sort of," Strange explains. "Your magic reached the mirror, but I don't think it reached through it. Try it again, but when you feel the point of your magic reaching the mirror, don't do anything. Instead, try to hold the form of the spell while concentrating on your destination: the mirror in your room."

For someone whose teaching strategy is mostly making things up on the fly, Strange is doing a pretty good job hiding the fact that this is all terribly spur of the moment.