Learn Finnish: Kysyin reittiohjetta ohikulkijalta. - I asked the passerby for directions.

This is the core rule for the verb kysyä (to ask). In Finnish, you don't ask someone directly; you ask from them. Verbs of asking demand the ablative case (-lta/-ltä) for the person being asked.

Why is the thing being asked for, reittiohjetta, in the partitive case?

The verb kysyä takes a partitive object for the thing you are asking for or asking about. Because you are asking for information rather than completing a physical action on a whole object, it naturally triggers the partitive.

Why is reittiohjetta singular when the English translation says "directions"?

The Finnish word reittiohje (route + instruction) is commonly used in the singular to mean "the way to get there". Using the plural partitive (reittiohjeita) would imply you were asking for several different routes or multiple unrelated sets of instructions.

How is the word ohikulkija built?

This is a compound of ohi (past/by) and kulkija (walker/traveler). The word kulkija itself comes from the verb kulkea (to go/wander) plus the -ja suffix, which works just like the "-er" ending in English to describe someone doing an action.