the woman to whom you were talking just now is Mary 's mother为什么要加 to 还有为什么用were 不是用was吗?

首先:用to是因为you were talking to ……这是不可省的，可以把它调到前面去！这样就调到whom前了
然后:用were，were是are的过去式，所以本来是you are过去式变为:you were……(I 'm 就变为 I was，而he is 是变为he was ，因为am和is过去式是was

The woman to whom you were talking is his mother。等于 The woman whom you were talking TO is his mother. 和谁谈话是talk TO sb. 不能说talk

我给你把主句从句拆看,你看下
the woman is Mary 's mother 这是主句 一点问题都没有吧
to whom you were talking just now 调整下语序,并请理解whom代替the woman
you were talking to whom（the woman）just now ,看到to是怎么回事了吗?talk to sb 和某人谈话；看到were是怎么来的了吗?根据主语you选are,根据just now改成were

主干是the woman is Mary's mother
to whom you were talking just now是个定语从句，修饰woman
to是和talk一起的，talk to sb
只不过可以把介词提前到关系代词whom前，是正式英语的用法，书面体，口语不这么说