How to mirror an EV3 motor? I want to get one EV3 large motor to copy the other. So if I move one half a rotation the other copies.
I am assuming that you want motor a to copy motor d and motor to is controlled manually(with your hands).  

You can use the motor sensor to sense the degree of movements of motor d, then use link it to how much motor a turns. However, as motor d will still retain the degree it has been turn, motor a will continue moving even after motor d has stopped(unless you turn back motor d). To counter this, you have to use the switch block and a bunch of compare blocks. You will have to make it so that if motor a's degree is the same or more than b's degree, the motor degree will reset. However, if I turn d counter clockwise, d's degree will be negative and the comparison(same or more than)will not work and it will keep spinning. Thus, I added another switch block that will compare motor a's degree and motor b's degree, this time resetting the motor's degree when it is a is equal to or smaller than d.  



This way what ever rotation done to d will happen to a.  

Hope this helps you!