obama made history by being elected as (the) first black president of the United States .为什么括号内要填the不是表示总统,班长一类的名词都不加冠词么

obama made history by being elected as (the) first black president of the United States .
为什么括号内要填the
不是表示总统,班长一类的名词都不加冠词么

表头衔,职位的名词只有在作表语,同位语,宾补时才不用冠词,这里是做介词as 的宾语的,不在可以省去冠词之列
序数词前要加the