Move (wrong category)

Hello,

So I am trying to make a simple game but I noticed something. What I wanted to do is when wp was equal to 0 I wanted to be able to have the A key turn the object to the left. So I added a keyboard sensor and an expression controller that was set to wp = 0 and a motion actuator. It worked but the cube would keep on moving to the left even though I was not pressing the A key. I want the object to stop moving when the key is released but I do not know how to do it. If anyone could help me, It would be greatly appreciated.
P.S. - I accidentally posted this same thing under the wrong category.

Thanks,
Matthew