“三变”:原句主语变为by短语,宾语变为主语,谓语由主动变为被动； “三不变”:原句意思不变, 原句时态不变, 原句主谓宾以外成分不变。 They are building nine parks. →Nine parks are being built. →How many parks are being built?

主动语态变被动语态: