By the time the police arrived ,they (kidnap)the poor boy successfully yet.
问题描述：
By the time the police arrived ,they (kidnap)the poor boy successfully yet.
答
hadn't kidnapped
由arrived知是过去完成时,又由yet知道要用否定.
整句话意思是,直到警察赶来时,他们还尚未成功劫持那可怜的男孩.