it is natural/necessary/strange/ a pity / possible +that从句 表惊奇、怀疑、惋惜、不满、理应如此.
without saying good-bye to us已完成,用完成时.
may表示可能性较小的状况下.
A. should snow 表示假设.在虚拟条件句中用以加强假设语气,用"should+do"指将来的情况,当"万一(会)"讲,表示对将来的虚拟.对过去的虚拟用had done sth, 对现在的虚拟用did sth, 对将来的虚拟用should do sth,或者were to do sth.所以不用 would snow