how long has she been helping her husband?---she was a young

选2 since 自从她还是个年轻女子开始.用于现在完成进行时
1.From then 本身就是现在完成时或现在完成进行时的时间状语,then 与she was a young woman重复
3.for要加时间段,而she was a young woman 是时间点
4.by then 用于过去完成时