It is adj. of sb. to do sth中的adj.跟sb.有关,

这个adj.是用来形容sb.的,

表示某人这么做真是太adj.了．

你会发现在这个句型中你把sb.和adj.提出来可以造个句sb. is(are) adj.

如It‘s very kind of you to help me.

把里面的sb.和adj.提出来可以发现you are kind是说得通的.

而It is adj. for sb. to do sth表示做某事对某人来说很adj.

这里的adj.是do sth.的属性

里面的sb.跟adj.没有直接联系

如It‘s difficult for you to deal with the problem.

4.The United Nations refugee agency has criticised plans by Greece to build a fence along part of its border with Turkey to keep out illegal immigrants.

to build a fence是动词不定式做后置定语，修饰plans。

在某些句型结构中可用动词不定式作后置定语。例如：

1) I have tons of letters to answer. 我有很多信要回。

2)She needed someone to take care of her baby. 她需要人照看她的婴儿。

3) They were the first to bear hardships, the last to enjoy comforts. 他们吃苦在前，享受在后。

在某些名词后可用动词不定式作定语。 例如:ability, anxiety, attempt, decision, chance, courage, right, determination, promise, ambition, drive, effort, eagerness, failure, inclination, intention, movement, need, opportunity, plan, reason, struggle, time, way, wish等等。例句：