这个高二英语选择、填空答案求解释

这个高二英语选择、填空答案求解释
1、With a large amount of work(A)the chief manager couldn't spare time for a holiday.
A.remaining to be done B.remaining to do
问：老师解释是后面的to do和with无关,和remain有关.为什么remaining这里不能解释为剩下,在这里做work的后置定语然后因为是主语做所以用with sth to do?
2、What you should do is（study）hard.
问：为什么填study而不是studying做动名词?
PS：case position 做定语从句先行词时后面用where还是when