Before mailing it in, please make sure that you enclose your reference letters
with your resume.
make是SVOC
that you enclose your reference letters with your resume是名词子句
sure是补语
书中的解释是O太长所以掉到后面，但是这种状况不是应该用虚主词it吗?
如下面这样
please make it sure that you enclose your reference letters with your resume.
请问这句话的文法是如何呢 ?谢谢^^