英语中什么叫做独立主格?

比如说，When I walked down the street,an accident happened.这是时间状语从句。Walked down the street,I saw an accident happened.是有非谓语的从句。而独立主格就是I walked down the street,an accident happened.就是避免主句与从句的主语不一致而在非谓语之前加上主语