someone had told the police that thieves would try to steal the diamonds\为什么要加had 直接 someone told ...

someone had told the police that thieves would try to steal the diamonds\
为什么要加had 直接 someone told ...

为了表明had told 这个动作先发于a few hours earlier这个一般过去时的时间,故用了过去完成时的had told,而只用told无法表达该动作的先发性.

因为told的宾语从句用的是过去将来时，所以主句表达的意思应该是过去的过去，所以用过去完成时，所以加had

因为钻石已经被偷，偷钻石是过去时，而有人告诉警察发生在这之前，所以是过去的过去，要用had过去完成时