Learn Finnish: Tämä on pakastin, jossa ruoan säilytys on turvallista. - This is the freezer in which food storage is safe.

The subject of the relative clause is säilytys (storage), which is an abstract noun representing an action. In Finnish, when the verb olla connects an abstract concept or an uncountable mass noun to an adjective, that describing adjective must be in the partitive case.

How is the noun säilytys formed?

It is derived from the verb säilyttää (to keep, preserve, store). Adding the suffix -s turns the verb into a noun that represents the action of that verb, much like "-tion" or "-ing" does in English.

Where did the "k" go in the word ruoan?

This is the genitive form of ruoka (food). Because of consonant gradation, the strong consonant k disappears when the closed syllable ending -n is added, leaving ruoan. You will also frequently see it spelled as ruuan, which is a fully accepted alternative.

If we wanted to say "in the freezer" rather than using the relative pronoun jossa, how would pakastin change?