it is probably that还是it is probable that哪个对还是都对?

都可以
比如 It is probable that he has forgotten our appointment.
很可能他是忘了我们的约会了。

It is probably impossible.
这几乎是完全不可能的。
看你怎么用了
希望你满意~

肯定是第二个，别误导人家
一样it is impotent that
it is good that 中间是形容词，不是副词

应该用It is probable that...,有个常用固定句型：It is+形容词+that/to do .probably 是副词，不符合！还要注意be动词（am,is,are)接形容词作表语，构成系表结构。副词多用于修饰实义动词

第一个

后者正确,主系表再引宾语从句.
前者用法如下但不用that连接
it is probably a mistake.
it is probably adequate in the circumstances.
It's probably the most important game.