Learn Finnish: Viallista osaa ei hyväksytä. - A defective part is not accepted.

This is how the present negative passive is formed. You start with the positive passive form (hyväksytään, meaning "is accepted") and simply drop the final -an or -än. Combine it with the negative word ei, and you get ei hyväksytä (is not accepted).

Why is viallista osaa in the partitive case here?

In Finnish, the object of a negative sentence must always be in the partitive case. Because the verb (ei hyväksytä) is negative, the thing being rejected (the defective part) takes the partitive endings.

Is viallista osaa the subject or the object of the sentence?

It is the object. In English, we translate this as "A defective part is not accepted," where "part" is the subject. But in Finnish, the passive is more like saying "(An unknown someone) does not accept a defective part." Because it receives the action, it acts as the object and follows the object grammar rules.

How do we get viallista from the basic adjective?

The basic form of the adjective is viallinen (defective). Remember that for words ending in -nen, you change the -nen to -s- before adding the partitive ending -ta or -tä. This gives us viallista.