比较下面两句,你的这个问题就解决了：
1 She says,"It happened a long time ago."(直接引语) ----- a long time ago 用于一般过去时
2 She said that it happened a long time before.(间接引语)-----a long time before 用于过去完成时
a long time before 在这里等于long before.,这里time是可数名词,所以前面要写上a------:a long time ago / before.