Learn Hardcore Finnish: Kartta auttaa turisteja suunnittelemaan reittinsä.

The verb auttaa (“to help”) typically takes its object in the partitive case to indicate an ongoing or incomplete action. Here turisteja is also plural because we’re talking about “(some) tourists” in general.

What role does kartta play in the sentence and why is it in the nominative?

Kartta (“the map”) is the subject of the sentence. In Finnish, the subject of a declarative clause stands in the nominative case.

Why is the form suunnittelemaan used instead of a plain infinitive like suunnitella?

After verbs like auttaa, you use the third infinitive in illative (the “-maan” form) to express “helping to do something.” So suunnitella becomes suunnittelemaan (“to plan”).

What does reittinsä mean, and why does it look so long?

Reittinsä breaks down into reitti (“route”) + -nsä, the third-person possessive suffix. Together it means “their route.” The suffix -nsä tells us the route belongs to the tourists.

Could we say suunnittelemaan reitin instead of suunnittelemaan reittinsä?

Not really. Suunnittelemaan reitin (“to plan a route”) might imply planning some unspecified route, but if you mean “plan their own route,” Finnish requires the possessive suffix: suunnittelemaan reittinsä.

Why aren’t there any articles like “the” or “a” before kartta or turisteja?

Finnish doesn’t have articles. Definiteness or indefiniteness is understood from context and case marking alone.

How do we know that the possessive suffix -nsä refers to the tourists and not to the map?

Possessive suffixes agree in person and number with the possessor. Here the suffix is third person plural (-nsä), matching turisteja (“tourists”), which is also third person plural.

What is the overall sentence structure in terms of Finnish grammar?

It’s SVO + infinitive complement: S (Kartta) – V (auttaa) – O(partitive) (turisteja) – Complement (suunnittelemaan reittinsä).
In English: “The map helps tourists to plan their route.”