Of course the majority of beginner violin students have the problem of their bows not moving straight (not moving parallel to the bridge). The problems comes to some students at various degrees; some have their bows all over the place and for others the problem is really minor...
This bow problem arises mainly from one of two reasons: either no proper wrist movement or too much shoulder movement. Either way, the result is mainly the same: bow not moving straight.
I've noticed over the years that explaining how to move their wrist and their shoulder and their arm properly and asking them to implement them is often futile! even though they understand what they should do, they can't implement it. This is largely irrespective of students age; students younger than 7 and older than 20 all have the same result.
The alternative approach is to ask them to focus (actually "stare diligently" :D) at the bow while playing and force themselves to keep the bow straight. I don't even have to explain how the arm and the hands should move! This surprisingly works much better. With this approach, students actually manage to keep the bow straight and gradually it is something they do effortlessly...success! Their arms and wrists move correctly..
I guess the reason is that our brains automatically have a much better understanding of the inner workings of our muscles. Just order your brain to keep the bow straight, and it will orchestrate your hand muscles to do it.
Anyhow, I really think that focusing on the end result when learning a certain technique leads to much better results than trying to explain and implement the inner workings of this technique. A similar experience with "trills" will come in another post very soon...
Have fun ;)
Mahmoud
t: @oasis9020
