A short list of things that will help you better understand Scandinavian folklore creatures. Some I've talked about before, others are new.
1) They are people. While they could turn invisible to humans they weren't considered spiritual creatures, but people with their own cultures. The troll culture, the elf culture, the gnome culture, and so on. They were believed to live normal everyday lives with their own conflicts and struggles, much like fantasy races in modern fiction.
2) They will treat you like you treat them. While some creatures are more dangerous than others, they always respond to respect with respect, kindness with kindness, and rudeness will be severely punished. This goes for ALL of them, even those popular media has taught you are either nice or evil.
3) They are connected with sex. Some like the huldra are inherently sexual, but even those that are not sexual in nature like gnomes still hold a stronger power over people who think a lot about sex, such as young people and those about to get married.
4) They don't care about gender. Don't think that just because you're a woman the elf girl won't ask you to dance with her, and the huldrekarl will be happy to take you to his home even if you are a man.
5) While most are magic users, it seems to drain them. Trolls and elves are very good with magic, while most others can only use it for a few minutes. Gnomes are so bad at it that a lot of stories involve them turning into calf or foal and suddenly changing back in front of humans because they couldn't keep it up.
6) While most of the creatures can be found in Norwegian, Swedish, and Danish folklore, they don't act exactly the same. A rule of thumb seem to be that the creatures are nicest in the south and becomes meaner the further up north you get.
Hope that helped those of you who have asked a lot about the nature of Scandinavian folklore.