【解説】
肯定文で主語，補語，目的語で使われる名詞（人）が「誰」であるかをたずねる疑問文を作るときに使われる疑問詞が「who」です。
| 主語（He）をたずねる | He plays soccer. → Who plays soccer? |
| 補語（Taro）をたずねる | He is Taro. → He is who. → Is he who? → Who is he? |
| 目的語（Taro）をたずねる | She likes Taro. → She likes who. → Does she like who? → Who does she like? |