"It is the capital city of the district."
Translation:Het is de hoofdstad van het district.
Have a read through this: https://www.duolingo.com/comment/3732938
In short, there are some rules but they can get complicated quick, so it is mainly a matter of learning it for each word. Some rules that always apply:
- Plurals always get de.
- Diminutives (singular) always get het
- Plural diminutives are still plural and hence get de