Mary told me that she had been to China （ ） .（two）教教我

Mary told me that she had been to China （ ） .（two）教教我
Mary told me that she had been to China （ ） .（two）
教教我

Mary told me that she had been to China （ ） .（two）教教我
这句话的中文意思是“玛丽告诉我她不曾去过中国”
“Mary told me”,即“玛丽告诉我”,已经用了过去式,
分句“she had been to China ”,她没有去过中国是“告诉”这个过去动作的过去,即过去的过去,所以用了过去完成时态.