正确的是：It's adj for sb to do sth。句式为陈述句，表示人对客观事物的决断，比较正式例句：It's necessary for us to have healthy eating habits.拥有健康的饮食习惯对我们来说很必要。
for的用法
在通常情况下，英语不用for doing sth来表示目的。如：他去那儿看他叔叔。
误：He went there for seeing his uncle.正：He went there to see his uncle.
但是，若一个动名词已名词化，则可与for连用表目的。如：
He went there for swimming.他去那儿游泳。(swimming已名词化)
注意：若不是表目的，而是表原因、用途等，则其后可接动名词。