a desk where he wrote letters
改为陈述句为:he wrote letters at a desk. 所以是“at which” 或者是“where” ,只有which是不对的。定语从句的连接词判断一般都可以用这种方法的,改为陈述句语序判断。
that his father used to sit in
这个好像不大对,应该改为which的,that是不能与介词搭配的
上句=A desk in which he write letter如果光是which缺少介词,不通。