The problem being discussed is very important,成分分析

“如果把being discussed看成是problem的定语的话,就应该翻译成正在被讨论的问题是非常重要的,”你这种理解是对的,但“看成动名词作主语,problem是动名词的逻辑主语”这种理解就不对了.
成分分析：
句子主干为：The problem is very important.问题很重要
全句应为The problem which is being discussed is very important.
which引导的定语从句可缩写为being discussed
如果说是动名词作主语,句子主干就变成：being discussed is very important.被讨论很重要,不合逻辑