英语翻译：there were more casualties than was reported.

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。