Learn Swedish: Kvällen då de vann matchen var underbar. - The evening when they won the match was wonderful.

Here, "when" points back to a specific time-related noun (kvällen). When "when" is used as a relative adverb to describe a noun, Swedish traditionally uses då. You will also hear när used this way in everyday speech, but då is the standard choice for connecting a clause to a time word.

Does this sentence break the V2 (verb-second) rule? Why is var so far into the sentence?

The V2 rule is still working perfectly! The entire phrase Kvällen då de vann matchen (The evening when they won the match) acts as the single subject of the main clause. Since that whole chunk takes up position 1, the main verb var correctly takes position 2.

What is the infinitive form of vann?

It is the past tense of the strong verb att vinna (to win). Because it is an irregular strong verb, it changes its vowel instead of taking a standard ending. The forms are vinna (infinitive), vinner (present), vann (past tense), and vunnit (supine).

Why is the adjective underbar and not underbart?