当然是 it was ；
than 在这里是连词,后面接 it was reported that there were ... ；
其实这一句的意思也可以这样表达：It was reported that there were some casulties, but actually there were more.；
what 在作为关系代词引导定语从句时,相当于“先行词 + 关系代词”；
在这一句当中,因为前面的 casuslties 是复数,what 则相当于 those which / that ,根据主谓语一致的原则,原句应为：there were more casualties than were reported；
句子的意思是：伤亡人员比报道的要多.也就是说，如果原句是there were more casualties than were reported，那么此处就用what？即该用what还是it仅仅靠was和were来判断即可？？？对，在这一句，是这样的。那这2句：There were more casualties than what were reported.There were more casualties than it was reported there were.完全等价？注意，是完全？若不完全，有何本质上的区别？谢谢！这两句话在意思上没有什么大的不同，但在语法上，结构是不一样的； There were more casualties than what were reported = There were more casualties than those which were reported ； those which were reported = the casualties which were reported = 报道的伤亡人员； the casaulties 在这里是先行词，定语从句中的主语，也就是关系代词 which 在数上面应该要与它一致，所以用 were；it was reported there were = it was reported that there were some casaulties = 据报道，有一些伤亡人员； 这里的 it 是形式主语，后面的 that 从句是真正的主语；it 做主语，应该用 was 。