Why should I connect the l289n motor driver wires to specific arduino pins only?
Pwm is just a digital out with extra fancy on off controls.  You can use any digital out if you do not need those extra features.
To just signal a single H bridge (1 motor of your l289n) for forward, reverse, stop, one only needs two digital outs.
I recommend you get the hang of your h bridge first - signaling it on a bread board with vcc (usually 5v but can be 3.3) and ground rather than the arduino.  Then once you understand the motor directions that result from your wiring, write the code accordingly.  This tends to result in:

fewer fried arduinos
code with functions and signals named after their intent (rather than having to mix forward and reverse)