• What do the individual circuits look like, by which the effects of the individual sources can be calculated?
  Which equivalent resistor must be used to replace a current or voltage source when calculating the individual effects?
• Where are the open-circuit voltages applied when looking at the individual components?

• substitute the current source $I_2$ with a short-circuit
• substitute the voltage source $U_3$ with an open circuit

The components can be moved in order to understand the circuit s bit better.

For the open circuit, no current is flowing through any resistor. Therefor the effect is: $U_{AB,1} = U_1$

(current) source $I_2$

• substitute the voltage source $U_1$ with an open circuit
• substitute the voltage source $U_3$ with an open circuit

Also here, the components can be shifted for a better understanding:

Here, the current source $I_2$ creates a voltage drop $U_{AB_2}$ on the resistor $R_2$ : $U_{AB,2} = - R_1 \cdot I_2$

(Voltage) source $U_3$

• substitute the voltage source $U_1$ with an open circuit
• substitute the current source $I_2$ with a short-circuit

Again, rearranging the circuit might help for an understanding:

In this case, between the unloaded outputs $A$ and $B$ there will be an unloaded voltage divider given by $R_3$ and $R_4$. On $R_1$ there is no voltage drop, since there is no current flow out of the unloaded outputs.
Therefore:

\begin{align*} U_{AB,3} = \frac{R_4}{R_3 + R_4} \cdot U_3 \end{align*}

resulting voltage

\begin{align*} U_{AB} &= U_1 - R_1 \cdot I_2 + \frac{R_4}{R_3 + R_4} \cdot U_3 \\ \end{align*}