答案应是：It's important that the librarian (should) make sure that the book （should）be returned in time.
1.此句有两个从句,第一个that the librarian (should) make sure,是it is important that...要求的虚拟语气,因此make sure 不用第三人称单数的形式.
2.第二个从句是make sure that.宾语从句,同样这里make sure 也要求虚拟语气,因此后面用 (should) be returned