1. None of this is nice for the people concerned, especially those with large student debts. 

—— concerned 是形容词，作后置定语表示“有关的”。

2. Not until the 1980's did some concerned citizens in Beijing started to find ways to preserve historical buildings from destruction. 

—— concerned 是形容词，作前置定语表示“关心的；关切的”。

【再次比较】

the problems concerned  有关的问题

the concerned educators  忧心忡忡的教育者们

3. as far as one is concerned：习语。“就某人而言”。如果细分的话，concerned 仍然是形容词，作表语表示 “有关的”。

4. concerning：现在分词或介词，“涉及；关于”。例如：

For more information concerning the club, contact Mr. Coldwell.

想了解更多有关该俱乐部的信息，请跟科德韦尔先生联系。