Learn Finnish: Pomo sanoo rahtia varastoitavan. - The boss says that the freight is being stored.

Varastoitavan is the present passive referative form (partisiippirakenne) of the verb varastoida (to store). It acts as a more formal shortcut to replace an että (that) clause. In this sentence, it literally translates the idea "that [the freight] is being stored."

How do you build the form varastoitavan from the dictionary verb?

You start with the standard present passive form of the verb. For varastoida, the passive is varastoidaan (is stored). You drop the final -an and add -a to get the participle varastoitava. Finally, you add the genitive -n ending which is always required for the main verb in this construction, resulting in varastoitavan.

If I want to use standard, everyday spoken Finnish instead of this advanced structure, how would I say this?

You would unpack it into a normal että clause with a regular passive verb: Pomo sanoo, että rahtia varastoidaan. The referative construction simply compresses että varastoidaan into varastoitavan.

Why is the word for freight (rahti) in the partitive case (rahtia)?