我想楼主的疑惑在于为什么不是confirms 而是confirm.
是这样的,这是虚拟语气.it is important that...中,that从句里的谓语动词要用原形,或者should加动词原形.important换成imperative,essential等形容词也要虚拟语气.
Collins 词典里有这样一个例句：
It was important that he rest.