求助：非谓语动词问题

Question

Staff remaining in the lounge should notify their immdiate supervisors before （returning） to their work.
为什么括号要填returning，书上说要填非谓语动词，但是连词后面不是应该有谓语动词吗

Answer 1

before,when,while等词后面可以直接用非谓语形式，相当于这些词引导的从句，是省略了相同的主语。这句话可以是：Staff remaining in the lounge should notify their immdiate supervisors before they returned to their work.用-ing形式，是该动词与主语之间是主动关系，如果是被动关系，则和其过去分词形式。He was surprised when (he was) asked to leave at once.追问

拜托追问一下，没省略之前动词就是谓语形式对吗？
那省略掉主语后，为什么留下的动词变成非谓语形式？