Martin hated --------- when his mother sent him to a boardin

that一般后面跟的句子是发生的具体的事件,
而题目中跟了一个when从句,是表示时间状态的,
所以Martin不是hated这个时间状态,
而是hated这个时间段发生的事”his mother sent him to a boarding school",用it是比较适当的.
如果要用that的话,句子可以这么改：
Martin hated that his mother sent him to a boarding school.