it's adj+to do还是doing?

问题描述:

it's adj+to do还是doing?

It's adj+to do sth.
在这个句型中,it是形式主语,代替后面的动词不定式.
例如:It's important to learn English well.学好英语很重要.那为什么我看到一个确实这样的it's always difficult being in……这个是动名词做主语。表示状态。可以解释一下吗?为什么?being是动名词了,例如:Being alone is his habit.独处时他的习惯。