副词从句简化为Ving补语后，如何判断是否保留连接词？

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While lying on the couch, the boy fell alseep.

副词从句因为已经简化，不再有主语、动词

while此处为连接词，若为其他连接词如何判断是否省略

asked Mar 8, 2019 in Questions about English Grammar by 118070300129huangyue (160 points) | 627 views