Finishing the work in an hour is easy for us to do.
在这个句子中, for us to do 是 the work 的定语, us 是 to do 的逻辑主语。为了避免主语和谓语动词(本句中是系动词 is )隔得太远,而把定语放在表语 easy 的后面。
这个句子也可以改成:
Finishing the work in an hour is easy for us.
或者
Finishing the work in an hour is an easy thing for us to do.