答案明确而简洁：turn 的过去分词是 turned。 作为规则动词，“turn”的过去式与过去分词形式一致，均通过在词尾加“-ed”构成，发音为 /tɜːnd/。
在实际应用中，turned 主要用于以下场景：
成时态中，它与助动词“have/has/had”搭配，动作的成或对现在的影响。例如：- She has turned the proposal into a detailed plan.
- By sunset, we had already turned back from the mountain.
被动语态中，turned 体现动作的承受者。例如：
- The old factory was turned into a community library last year.
- The keys are turned in the lock every night.
作为形容词时，turned 描述事物的状态。例如：
- He found a turned page in the notebook.
- The turned leaves indicated the arrival of autumn. 掌握 turned 的用法，需意其与原形“turn”、过去式“turned”的语境差异。论是表达“转变”“转动”还是“变得”，turned 都是“turn”在过去分词形式下的唯一正确呈现。