Question

The old man has three sons,...

sons,but none of ____ is a doctor. 为什么不能填whom？

Answer 1

原句The old man has three sons, but none of them is a doctor.中them不用用whom替换哦。
分析：
一，本句话是连词but连接的表示转折关系的并列句。换言之the old man has three sons和none of them is a doctor是并列关系，其中的人称代词宾格them代替three sons。
二，whom做关系代词时要用在定语从句中。如果在本句话中使用whom，则原句可以改为:
The old man has three sons, none of whom, however is a doctor.
该句是含有非限制性定语从句的复合句，whom是关系代词，代指three sons。复合成分none of whom在从句中作主语。
（希望对您有帮助哦！）

Answer 2

The old man has three sons, but none of them is a doctor.

后面去除of them是一个完整的句子，没有宾语从句，或者宾语。

of them 是用来限定none的。所以用them不用whom

Answer 3

but none of__them__is a doctor.
因为前面有but说明是并列句，后面不是从句，是独立的句子，用人称代词，不用关系代词。