我的想法是,这个地方be about to 是一个搭配,就是将来时的一种表示方法,他要改变想法了.所以其实“必定”要修饰的是change这个动作,副词修饰动词.就像:he will definitely change his mind一样的,你就想把will 用be about to 替代就可以了,he was definitely about to change his mind.