Why is voltage constant in a parallel circuit but not in a series? Why is voltage constant in a parallel circuit but not in a series? Also what forces are affecting an electron in a circuit?
 A: Let's start with the second question because it is easier to answer:
The forces come from the electric field induced by the voltage difference at the battery poles (in a DC circuit, in an AC circuit it usually come's from Faraday's law but lets not get into it. Actually, for simplicity, I'll explain everything in a DC circuit).
Now, for your first question, I will answer twice: with math and with intuition, and you'll judge which you prefer.
The math: Kirchoff's law says that the voltage drop between a point and itself after 'doing a loop' must be 0. In other words:
$$\oint \vec{E}\cdot \vec{dl} = 0$$
Lets look at this circuit I found in Google Images for example: 
If I take the loop $7\to6\to3\to2\to7$ then the voltage drop should be $0$.
Mathematically this can be written like this:
$$V_{7\to6}+V_{6\to3}+V_{3\to2}+V_{2\to7}=0$$
And because points 2,3 and 6,7 are connected with wires with negligible resistance, the voltage drop between those points is zero:
$$V_{7\to 6} = V_{3\to 2}=0$$
Which leaves us with Kirchhoff's law looking like this:
$$V_{6\to3}+V_{2\to7}=0$$
The voltage drop from $2$ to $7$, $V_{2\to 7}$ is just minus the voltage drop in the opposite direction, $V_{2\to 7} = - V_{7\to 2} $, So we get:
$$V_{6\to3}+V_{2\to7}=V_{6\to3}-V_{7\to2}=0\Rightarrow$$
$$V_{6\to3}=V_{7\to2}$$
Which proves what we wanted: two resistors in parallel have the same voltage drop.
You cannot make a similar argument for the resistors in series because there is no close loop there for applying Kirchhoff's law.
The intuition: Voltage drop is like difference in height and a resistor is like a slide with friction that doesn't let you speed up, just transports you from a high altitude to a low one, losing all that energy to friction with the slide.
Now a battery, it creates a voltage drop, think of it as being a stairway up with constant length, and you let people go from top to bottom and complete the cycle only if you connect a slide (resistor). 
Do you see how putting 2 slides in parallel from top to bottom, they take you down the same height but it doesn't have to be so if you connect them in series? If they are the same they will have the same height drop, but if not, they don't have to be. Did it help you get it?
