We have a teaching philosophy that is unique. Most sailing schools will do a "ground school" for hours before the students get on a boat. The idea is to explain how it all works, so the student is prepared intellectually for the experience. Our approach is to get them on the water right away and have them just do it - steer the boat, tack, and get the feel for it.
I believe that our approach is correct, up to a point. You need some practice before the sailing theory makes any sense. And my experience as an instructor is that a beginning student will do just fine steering beam reach, tacking, even steering close-hauled without any discussion of how it all works.
The turning point is when they try to control the tiller and the main sheet at the same time. Lots of confusion, including understanding wind direction. Tacking from beam reach to beam reach and seeming to lose the wind (because the sails are trimmed too tightly).
This is the point where they really need an understanding of how it all works. And that's hard to do on the water. You need an image to show how it works. I believe it's all pretty simple (or at least
can be explained simply), with a visual from above.
The key concept is that upwind (beam reach and higher) the sail is a wing, it works just like the wing of an airplane. Now an airplane wing has to be at a certain angle to the wind, or it doesn't work. Same with a sail. So when you're trimming the sail going upwind, you're keeping the sail at the same angle to the wind all the time.
Let's see what this looks like from above on several points of sail - close hauled, close reach, and beam reach (click on the diagram to start the animation):