The Dutch passive voice (lijdende vorm) is formed with worden (to become) + past participle for ongoing or processual passives, and zijn (to be) + past participle for state or resultant passives. Process passive: De brief wordt geschreven (The letter is being written — process in progress). De vergadering wordt gehouden (The meeting is being held). State passive: De deur is gesloten (The door is closed — result/state). Het werk is gedaan (The work is done — completed result).
Perfect passive: The perfect of the worden-passive uses zijn: De brief is geschreven (The letter has been written — zijn + participle, not worden + worden). This is potentially confusing because zijn is used both for state passives and for perfect worden-passives. Context usually clarifies: De deur is gesloten can mean the door is closed (state) or the door has been closed (perfect passive). The imperfect passive: De brief werd geschreven (The letter was being written/was written).
Expressing the agent: the agent (who performs the action) is introduced by door (by): De brief wordt door de secretaresse geschreven (The letter is being written by the secretary). Door introduces the agent in all passive constructions. The agentless passive (most common in Dutch) simply omits the agent: Er wordt hier niet gerookt (Smoking is not done here — literally: there is not smoked here). This er + wordt/werd construction is the most frequent passive form in everyday Dutch and is used extensively for general rules and notices.