passive voice幾時用吾同form?? 急!!!

He was killed by the robber = past tense in passive voice
The robber killed him = past tense in active voice
The bridge has been built by the road department = present perfect tense in passiv voice. It means the bridge has been built or something has been done.
The road department has built the bridge = present perfect tense in active voice. It also means the bridge has been built or something has been done.

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The robber was killed by the policeman. (搶匪已被殺 - 過去式)
The flyovers have been built ( 天橋已經被興建 - 現在完成式)