表达 “rule the roost” 相当于汉语里说的 “当家作主”，指 “在一群人或一家人当中最有权威和影响力、可以作决定的那个人”；这个人通常要作出所有重要的决定。

例句
His girlfriend rules the roost, so he’ll have to check it’s OK with her first．
他的女朋友平时作主，所以他得先和她确认一下这件事。

My mum is always in charge of important decisions. She rules the roost.
一直以来，我妈妈负责作重要的决定。家里她作主。

I always thought Dave ruled the roost in his house, but I think his dog is really in control!
我一直以为戴夫是他家作主的那个人，但我看他的狗才是真正管事的！