I move two tab pages in evolution to constrain page.

Normally, we only put relations which will changed with time in evolution page.

like the rate equation. And the solver will use suitable to calculate the best value for next time step.

I also change the solver from midpoint method to Runge-Kutta 4th order method.

You should use this one for most of the simulation. Midpoint method is only good for constant acceleration or similar relations. It is not good for other cases (numerical error will accumulate).

I also change code so that the "play" button will toggle between "play" and "pause".

You sould be albe to create a two level system from this one.

Add one more equation in evolution page and rearrange GUI elements to model two level tanks!

Good job! Keep going! :D