句子结构:
(1)肯定句: 主语＋be动词(was，were)＋其他
           ---She was a student two years ago.两年前她是一个学生.
    否定句: 主语＋be not(was not, were not)＋其他
           ---She was not a student two years ago.两年前她不是一个学生.
    疑问句: Be动词(Was/Were)＋主语＋其他
           ---Was she a student two years ago?两年前她是一个学生吗?
(2)肯定句: 主语＋动词的过去式＋其他
           ---She ate three apples yesterday.昨天她吃了三个苹果.
    否定句: 主语＋did not＋动词原型＋其他
           ---She did not eat three apples yesterday.昨天她没吃三个苹果.
    疑问句: Did＋主语＋动词原型＋其他
           ---Did she eat three apples yesterday?昨天她吃了三个苹果吗?
注意:wasn't 是 was not 的缩写.weren't 是 were not 的缩写.didn't 是did not 的缩写.
常用的时间状语:
yesterday昨天     two days ago,three days ago(两年前,三年前)等等.
long ago很久以前    last + week(year, night, month…)上一周,去年,昨晚,上个月等等.
 in 1989,in 2008等等.
a moment ago 刚才,just now 刚才.