who is the boy with whom you were talking a moment ago.中with

后面一部分可还原为you were talking with him a moment ago.him做宾语,变成从句后当然是用其宾格啦~这是我的理解

whom是talking的宾语，所以用宾格

定语从句，talking是不及物动词，关系代词前要加介词

定语从句。
从句修饰the boy,因为whom这个位置在从句中做宾语，而且把介词with提前了，这里who就得用whom了。