1.in the tree指不是树上生长的叶子,花.指是树上生长的叶子,花时用 on the tree.
2.在世纪前用in ,in the 21st century
3.在年,月,星期,上午,下午,晚上前用in,in 2008,in May,in a week,in the morning ,in the afternoon,in the evening
4.在某一天前用on ,on Monday,on May 1st
5.在点钟前用at,at 8:00