Mary came to China three years ago.[改为同义句] Mary has [ ] in China since three years ago.

Mary came to China three years ago.[改为同义句] Mary has [ ] in China since three years ago.

been

come 是终止性动词，它的现在完成时肯定式不能与表示一段时间（FOR， SINCE 等短语）连用，故要改为持续性动词，COME 改为BE IN ，而BE 的过去分词是been追问COME 改为BE IN 【为什么？请说明白】追答因为去到某地，就在某地呀，since three years ago.是现在完成时的标志，而现在完成时的结构是have /has +动词的过去分词，BE的过去分词是 been，故用 has been in

been
这不是改同义句，而是时态的改变
原句为一般过去时，改后的句子是现在完成时追问那为什么用been？追答现在完成时的标志是 have/has done
be的过去分词是been

Mary came to China three years ago. =
Mary has [been ] in China since three years ago.
have been in 表示：在某地待着。
祝学习进步，有不会的可以再问我。(*^__^*)追问那为什么用been？追答has/have 后面接be动词的过去分词形式，构成一般完成时。
have been in 表示：在某地待着，可以与段时间连用。