Question

It's 十adj 十doing

Answer 1

按道理来说 一般+to do的形容词比较多 大多都是做什么很怎么样的意思

但有一些少数固定搭配的后面+doing 
如：
It is no use doing sth.
It is a waste of time doing sth.
It is useless doing sth.
It is no good doing sth.

不过像nice，两种都可 

it's nice doing sth.
做了某件事真好.(事情已经做完了)
It's nice to do sth.
做某事真好(没有做,即将要做)
例:
It's nice helping you.帮助你感觉真不错(已经帮助了)
It's nice to do the job.干这份工作会很不错的(即将开始干)

这也是to do 跟doing 的一种区别 不仅仅只是it is +adj.这个结构中.