Learn Finnish: Turvallisuuden takia pomon on lukittava ovet aikaisin. - Because of safety, the boss must lock the doors early.

This sentence uses the Finnish obligation structure to say someone must do something. In this pattern, the person who has to do the action is called the genitive subject because it always takes the genitive case (pomon). This is followed by the verb on and the passive present participle (lukittava).

How exactly is lukittava formed from the verb lukita (to lock)?

Lukittava is the passive present participle. To build it, you take the passive stem of the verb. For lukita, the passive form is lukitaan, so the stem is lukitta-. Then, you simply add the participle ending -va or -vä. Together with on, it creates the meaning 'must be locked' or 'must lock'.

Why does turvallisuus (safety) change to its genitive form, turvallisuuden, at the start of the sentence?

This change is triggered by the word takia (because of), which is a postposition. In Finnish, most postpositions require the noun that comes right before them to be in the genitive case.

If the boss is locking the doors, why is the object ovet (the doors) in the nominative plural rather than a partitive form?