than 在這裏是連詞，後面接 it was reported that there were ... ；
其實這壹句的意思也可以這樣表達：It was reported that there were some casulties, but actually there were more.；
what 在作為關系代詞引導定語從句時，相當於“先行詞 + 關系代詞”；
在這壹句當中，因為前面的 casuslties 是復數，what 則相當於 those which / that ，根據主謂語壹致的原則，原句應為：there were more casualties than were reported；
句子的意思是：傷亡人員比報道的要多。