it is really a strenuous thing

it is really a strenuous thing时,really为句子的状语,而strenuous thing为名词词组；
改成it is a really strenuous thing也成立.此时,副词really修饰形容词strenuous,really strenuous thing为名词词组.
不定冠词置于名词词组前,若副词是名词词组的一部分,不定冠词在副词前.
这样,第二句应该可以理解了.It is an ecologically friendly product时,副词ecologically修饰形容词friendly构成名词词组.