The Dutch passive is formed with worden (to become) plus a past participle for actions in progress: De brief wordt geschreven means The letter is being written. For completed states — where the result of an action is described — zijn is used: De brief is geschreven means The letter is written as a current state. In the past tense: De brief werd geschreven (was being written) versus De brief was geschreven (was written, the resulting state).
To introduce the agent — the doer of the action — use door: De brief werd door de secretaresse geschreven. However, in many passive sentences the agent is omitted entirely. Official and bureaucratic Dutch loves agentless passives: Er wordt verwacht dat (It is expected that), Klanten worden vriendelijk verzocht (Customers are kindly requested). These make the text sound authoritative and impersonal.
A distinctive Dutch passive is the impersonal passive with er as a dummy subject: Er wordt hier veel gefietst. This construction has no real subject and describes general activity, not a specific action. It is extremely common in Dutch and has no direct English equivalent. The active equivalent would be Men fietst hier veel, but the er-passive sounds far more natural in everyday Dutch conversation and writing.