it is adj + doing or to do?会的请帮解答

it is adj + doing or to do?会的请帮解答
Q1.
It is useful/useless/nice/good/interesting/expensive (to do / doing?)
这种的it is 加 adj 後面的到底用哪种正确?
似乎网上也很多人搞不太清楚有些说只能用 to do sth有些说只能用doing sth
有些说两种都可以?因为有人说什麼是正在做用doing 还没做用to do?看不懂
请教真正会的告知下不错再加分 不会的请勿乱回答 转贴 xie xie!
Q2.
还有这种
It is fun 这个後面是doing sth还是 to do sth?也搞不清楚

It is + adj. + to do something结构
　　句中it是形式主语,真正主语是动词不定式to do something.因此也可以表达为to do something + is +adj..
　　e.g. It is amazing to drive past fields which seem to go on forever or whole hills covered with neat rows of grapevines.
　　本句形式主语为it.真正主语to drive…grapevines移到了句子后部,使句子变得平稳,不致主语太长.如果把真正主语放在前面,那么整个句子就显得头重脚轻.因此,就用it作为先行主语.
　　这种结构还有两种动词不定复合结构.
1. It is + adj. + for somebody to do something
2. It is + adj. + of somebody to do something