He was elected --------------(to be a monitor/monitor)

He was elected --------------(to be a monitor/monitor)
He sat there ,-------------(read)a magazine.为什么用reading而不用read过去分词
Th
e ground is covered with ---------------(fall) leaves.为什么用fallen而不用falling
When he arrived, he found ---------------(所有的人都走了)为什么用all the people gone而不用all the people had gone.