"The duck swims to the girl."
Translation:Ankan simmar till flickan.
The girl is a specific point because "till" is being used, and the reindeer is a certain direction because "mot" is being used.
Ankan simmar till flickan - In this sentence, the duck is swimming to the exact location of the girl.
Vargen springer mot renen - In this sentence, the wolf is running towards the reindeer, and not to the specific location of the reindeer.
"Vargen springer till renen" and "Ankan simmar mot flickan" are both "correct" sentences, but they have different meanings from the two sentences above.
You can get the list of prepositions with brief translations here: https://en.wikibooks.org/wiki/Swedish/Prepositions and get a more detailed explanation of each at Wiktionary: https://en.wiktionary.org/wiki/Category:Swedish_prepositions
I really do suggest reading the Wiktionary entries for the prepositions you come across. There are a lot of example sentences.
I wish the Wiktionary entry for hos had the French word chez. Most English speakers know that word.