It is important that.一定是虚拟么?

对的!
这是虚拟语气特殊形式之一:
It is＋good(应该的),wrong(不当的),important(重要的),necessary(必须的)＋that＋主语＋should＋原形动词,should可以省略.
It is important that we (should) learn English well.
学好英语是重要的.
误用:
中文：他这么做是重要的.
(误)It is important that he will do it.
(正)It is important that he do it.
It is important＋that引导的<名词从句>的<动词>要用should加原形动词,should可以省略.