This is the 71st most frequent Finnish word.
ruotsin
The Finnish word "ruotsin" means "Swedish" (genitive form of *Ruotsi*).
Here, 'ruotsin' is again the genitive of 'Ruotsi', used to describe what sort of grammar it is.
Meidän pitää oppia ruotsin kielioppi.
We need to learn Swedish grammar.
In this context, 'ruotsin' describes possession, specifying the citizens belonging to Sweden.
Ruotsin kansalaiset tunnetaan ystävällisyydestään.
The citizens of Sweden are known for their friendliness.
In this sentence, 'ruotsin' is the genitive form of 'Ruotsi' (Sweden), indicating possession or other relationships, here specifying the type of language.