Learn Dutch: Was ik maar niet verkeerd gereden! - If only I had not driven the wrong way!

This is a verb-first (V1) exclamation used to express a strong wish or regret. By putting the verb first and adding the modal particle maar, the structure changes from a normal statement into an 'If only...' wish.

Why does this use Was (zijn) as the auxiliary verb instead of Had (hebben) with gereden?

The verb rijden takes hebben when you are talking about the general act of driving. However, when you mention a specific direction, destination, or a change of location—in this case, driving the wrong way (verkeerd)—it must take zijn. Because this expresses a past regret, the auxiliary becomes was.

What is the exact role of maar in this sentence?

In these verb-first exclamations, maar is the essential ingredient that turns a simple question structure into an 'If only...' wish. Without maar, Was ik niet verkeerd gereden? would just be a question meaning 'Had I not driven the wrong way?'. You could also use the particle toch to achieve a very similar effect.

Does verkeerd gereden literally mean 'wrongly driven'?