Learn Danish: Drengen ser en lang frakke. - The boy sees a long coat.

Because it is describing en frakke (a coat), which is a singular en word (common gender). When an adjective comes between en and a noun, it stays in its basic dictionary form. If it were describing an et word, it would get a -t at the end (like et langt bord), and if it were plural, it would get an -e (like lange frakker).

How do we know to use en before lang frakke instead of et?

The article you use always depends on the noun itself, not the adjective next to it. Since the word for coat is an en word (en frakke), you must use en. The adjective lang just slides right in between them!

I remember that dreng means "boy". Why does it say Drengen?

This is a quick refresher! In Danish, to say "the", you attach the article directly to the end of the noun. Since dreng is an en word, adding -en to the end makes it drengen (the boy).

Can ser translate to "is seeing" as well as "sees"?

Yes! Danish doesn't have a separate "-ing" tense for things happening right now. The present tense verb ser covers both "sees" and "is seeing" depending on what makes the most sense in English.