第一题,know后面部分为宾语从句,what提问的是supper的食物,原句there was **（某种食物）for supper that day.对这种食物用what进行提问,即what was there for supper that day?
放在宾语中,即I don't know what there was for supper that day.
第二题,regard sth.as.(+adj.)是固定搭配哦.