describe and explain how the concentrations of A,B,C,D change from the time when A and B first combine....?

Question

....to the point at which equilibrium is established for the reaction A+B<-->C+D

Answer 1

When A+B first combine, thee is plenty of opportunity for them to collide and react. So the reaction goes quickly. As they get used up, the forward reaction slows down.

Since this is a reversible reaction, the products now have enough to start colliding with each other and forming the old reactants

And so it goes, back and forth until the rate of the forward and backward reaction are the same. This is then called dynamic equilibrium