Learn Danish: Atleten som præsterede bedst, vandt løbet. - The athlete who performed the best won the race.

Here, bedst is an adverb describing how the athlete performed (the verb præsterede). When an adverb is in the superlative form, it usually ends in -st without an extra -e. If it were an adjective describing a definite noun, it would be den bedste atlet (the best athlete).

Could I use der instead of som in this sentence?

Yes! Because the relative pronoun acts as the subject of its clause (who performed best), both som and der are correct. Remember that der can only be the subject, while som can act as either the subject or the object.

Why is there a comma right before the verb vandt?

In Danish, you typically place a comma to separate clauses. The phrase som præsterede bedst is a relative clause inserted into the sentence. The comma shows that the relative clause has finished, and the sentence is returning to the main clause and its verb, vandt.

Why use præsterede instead of another word for performed, like optrådte?

The verb at præstere means to achieve a result or perform a task, which is perfect for sports, work, or tests. If the person were performing artistically in a play or playing a guitar on stage, you would use at optræde instead.