This is the 8th most frequent Finnish word.
suomen
"Suomen" means "of Finland" or "Finnish" (possessive/genitive form).
In this sentence, 'suomen' is the genitive case of 'Suomi' (Finland) and refers to the Finnish language as a possession or attribute.
Rakastan suomen kieltä.
I love the Finnish language.
Here, 'suomen' describes the nationality, relating to Finland, as an adjective.
Hän on suomen kansalainen.
He/she is a Finnish citizen.
In this example, 'suomen' is used in the genitive case to define 'kieliopin sääntöjä' (the grammar rules) as belonging to or associated with the Finnish context.