Learn Finnish: Käymme huoltoasemalla, koska tarvitsemme polttoainetta. - We'll visit the gas station because we need fuel.

This is because of the verb käydä (to visit). Unlike verbs of motion such as mennä (to go), käydä requires a static location case. You must use either the inessive (-ssa / in) or the adessive (-lla / at). Think of it as making a visit at the gas station rather than traveling to it.

Why does huoltoasema take the adessive ending (-lla) rather than the inessive (-ssa)?

In Finnish, places that are open areas, stops, or stations traditionally take the adessive (-lla / -llä). Just like you wait asemalla (at the train station) or pysäkillä (at the bus stop), you visit huoltoasemalla (at the gas station).

Why is the word for fuel (polttoaine) in the partitive case (polttoainetta)?

Polttoaine is an uncountable mass noun, like water, coffee, or rice. When an uncountable substance is the object of a verb, which in this case is the thing we need (tarvitsemme), it takes the partitive case to indicate an unspecified amount (some fuel).

Tarvitsemme comes from the verb tarvita (to need). Where does the -se- in the middle come from?