Learn Swedish: Eftersom han inte väjde för fotgängaren, så underkände inspektören honom. - Since he did not yield to the pedestrian, the inspector failed him.

The word så acts as a link between the initial subordinate clause (Eftersom...) and the main clause. It is a resumptive adverb. While it is not strictly mandatory—you could remove it and just say ...fotgängaren, underkände inspektören honom—using så is extremely common in Swedish to make the cause-and-effect flow more smoothly.

Why is inte placed before the verb väjde in the first half of the sentence?

This is a classic example of the BIFF rule. Because Eftersom han inte väjde för fotgängaren is a subordinate clause (a clause that cannot stand alone as a complete sentence), sentence adverbs like inte must be placed before the verb. In a normal main clause, inte would come after the verb.

How does adding så affect the V2 (verb-second) rule in the main clause?

It does not change the V2 rule at all! The entire first clause (Eftersom han inte väjde för fotgängaren) acts as the first foundation block of information. Whether you add så to the end of that block or not, the main clause verb underkände must still take the second position. This forces the subject inspektören to come after the verb.

What is the base form of väjde, and why is för used with it?