Learn Danish: Han var en meget genert dreng dengang. - He was a very shy boy back then.

The adjective genert (shy) natively ends in a 't' in its base dictionary form. Since dreng is a common gender (en) word, we just use the base form without adding or removing anything. For et words, it also stays genert because Danish adjectives ending in -rt generally do not take an extra 't'.

How does dengang relate to da (when) from this lesson?

Dengang means "back then" or "at that time." It specifically refers to a past timeframe, which connects perfectly to this lesson: you must use da for "when" precisely because you are talking about an event in the past. Danes often combine them into dengang da... (back when...) to emphasize a specific time in the past.

Why is dengang placed at the very end of the sentence?

In Danish main clauses, time expressions usually go at the very end. If you want to emphasize the time, you can move it to the front, but remember to flip the subject and verb to keep the verb in the second position (the V2 rule): Dengang var han en meget genert dreng.

Could I use dengang if I am talking about a time in the future?

No, dengang strictly means "back then" in the past. This perfectly mirrors the rules for "when": you use da for the past and når for the present or future. For a future "at that time", you would instead use the phrase til den tid.