"She is reading a newspaper."
Translation:Hun leser en avis.
"den" (m/f) and "det" (n) have several uses.
They can function as third person singular impersonal pronouns, like "it" in English, as determiners, like "that" in English, and are also used with definite nouns when they're coupled with an adjective:
"Den blå bilen" = "The blue car".
"Det" also functions like a dummy/formal subject in sentences like this one:
"Det regner." = "It rains"
You're not expected to know all of this so early in the course, so don't worry if it seems a bit confusing. It'll fall into place eventually. :)