I have to calculate the resistance R_AB, R_AC and R_CD. But I'm confused because there are two different terminal, one on the left and one on the right. Can someone give me a hint how to solve this?
Not sure why are you confused. For R_AB there are only two terminals, both on the left. For R_AC one is on the left and one on the right. For R_CD both are on the right.
You should just ignore unused terminals and solve the schematics using the rules of parallel and series resistors. Or Kirchgoff / Ohm laws.
For example for R_AB the C is ignored, you can erase it to the closest junction (but leave the resistors 10 and 10 connected). For R_AC you erase both B and C and then erase the 30 resistor since its bottom terminal is not connected anywhere.