举几个例子来说明问题
1,I'm reading a book.表示正在进行;
2,The road,making its way straight,was damaged.表示主动关系;
3,I decided to go on the trip yesterday.表示过去;
4,The paper printed by the teacher was lost.表示被印刷;
5,I was to go upstairs when he saw me.表过去将来;
6,To make it clear,I wrote a letter to him.表目的.