"He assumes, that we do not know anything."
Translation:Han antager, at ikke ved noget.
It's because "at vi ikke ved noget" is what is called a "ledsætning" in Danish. Basically, because "Han antager" is already a full sentence with a subject and a verb, the following part of the sentence inverts the adverbs to come before the verb. In this case, the adverb "ikke" comes before ved. This indicates that the ledsætning is a continuation of the thought and not a new sentence. Hope this helps!