The man preparing documents is my former schoolmate.
解答：prepare为动态动词，动态动词的现在分词作后置定语，只能改为“进行体的定语从句”作后置定语。
The man who is preparing documents is my former schoolmate.【正确】
The man who prepares documents is my former schoolmate.【错误】
备注(1):动态动词的现在分词作后置定语，只能改为“进行体的定语从句”作后置定语。
The girl singing in the room is Mary.
=The girl who is singing in the room is Mary.
备注(2):静态动词的现在分词作后置定语，只能改为“一般体的定语从句”作后置定语。
Tom needs a room facing south.
=Tom needs a room which faces south.
【1】The man who prepared documents is my former schoolmate.
备注：定语从句“prepared”为过去式，描述“过去事件/过去习惯”主句(is)为一般现在时态，描述“主语现在的状态”
【2】The man who prepared documents was my former schoolmate.
备注：定语从句“prepared”为过去式，描述“过去事件/过去习惯”主句(was)为一般过去时态，描述“主语在过去的状态”
【3】The man who prepares documents was my former schoolmate.
备注：定语从句“prepares”为一般现在时态，描述“现在的习惯”主句(was)为一般过去时态，描述“主语在过去的状态”
