将本句改为陈述句,相信容易理解得多：
It is an important work.显然work是不可数名词,不能加不定冠词an/a,不过换成定冠词the会更通顺.
本句应改为：How important the work is !
或者我们将work换成可数名词job代替：What an important job it is!