when引導的是特殊疑問句。
特殊疑問句的結構是:
特殊疑問詞(who what which when where why how等)+助動詞或情態動詞(do does did have had will can may must等 )+主語+謂語動詞原形+其他成分
例如:When do you want to visit the famous city?
人稱和時態變化都體現在助動詞上。
因爲when作疑問副詞,動詞要用第三人稱單數,When後面必須接助動詞do,因爲主語是第三人稱單數,所以do必須要用does 形式。