Flyback diode

I understand the need for a flyback diode to protect the circuit when a DC motor is used. But what I don't understand is why it must be connected in parallel? Why is it not possible to connect the diode in series? If it only lets through the flow in one way, then it must be, IMHO, working in series too (it cannot flow back)? Or am I wrong with this (probably I am)?

I just started electronics, so sorry for any beginners mistakes...

Or am I wrong with this

Yes you are. The idea is to short out the energy held in the coil. Just stopping the flow in one direction is still going to leave a damagingly high voltage in your circuit.

Do you mean that the voltage has no where to go, and therefor stays in the circuit? But the flow is not able to reach other components in the circuit, or is my assumption wrong about this? What paths is it able to travel?

Could you please explain it more thoroughly...

A diode have what is called a reverse leakage, that is when it is b locking current flow in the other direction it is actually passing some small amount. This means that this reverse voltage is not isolated from the rest of your circuit. Therefore it is causing that reverse voltage to exist on the supply rails amongst other things.
You don’t have to have much if any current to damage a semiconductor with a high voltage. For example there is very little current in a static electric discharge and that can blast holes in a semiconductor.
So just restricting the current that will flow is not going to protect anything.