Have you tried reading the Tips Notes for the lessons? (they might not work if you're on mobile).
https://www.duolingo.com/skill/sv/Basics-2 for instance
If we compare to English:
en pojke = a boy
pojken = the boy
It's always en pojke – you can never use ett with this word, that would just be wrong.
Does that help?
Historically, we had three genders just like German. But the masculine and feminine genders collapsed into what is called common gender, we usually call those words en words. The other gender is neuter, or ett words.
As you could probably guess, this means that most (but not all) nouns that denote living beings are en gender nouns. There are some tendencies, but basically you just have to learn the gender along with each word.
For pojke, it is
en pojke a boy
pojken the boy
pojkarna the boys
PS there are Tips & Notes texts at the beginnings of some lessons that explain things like this. Like here for instance: https://www.duolingo.com/skill/sv/Plurals