may i come home late today?no,you____.You will have a test tomorrow.

mustn't
记住一个顺序,这类问题就很好解决了
can may must need
在初高中经常遇到情态动词提问的句子,照上面的顺序,如果用前一个提问就用后一个回答.
如本题用 may 来提问,就用后面的 must 来回答.
例如：
Must I go there tomorrow?
否定回答：No,you needn’t!
补充：如果是couldn't的话应该用could提问..这是固定搭配的..
有的地方虚拟语气里会用到,但此处不是.
遮挡阳光的话为：keep the sun off
如下例：Then are you carrying the umbrella to keep the sun off you?
“那你拿伞是为了遮太阳吗?”
-- 来源 -- 初级英语100(1) - Part 43 Better Be a Stupid Man
希望有所帮助