It is much easier if you know how your own mother language works, instead of trying to understand "grammar logic" of foreign language with your own understanding.
Let's look at some example :
the sky [is] blue ：天空[是]蓝(色)的
the sky [is] crystal blue : 天空是透蓝的
The air [is] bad : 空气[很]差/坏
he [is] bad : 他[很]坏. 他[是]个坏人. 他[很]差劲
he [is] naughty : 他[很]顽皮
he [is] (a) bad guy : 他[是] (个) 坏蛋/坏人
If you think the confusion is cause by mandarin connector, it is half true.
I.e., the sentence
The air is bad carry out (ambiguous) relative comparison . Mandarin use 很 to explicit brought out relative comparison attributes.
He is tall , is relative. It carry out additional comparison message e.g. "He is tall, compare to his sibling". 他很高，和他兄弟姐妹比较的话。
是 used for affirmative, or agree
He is (a) USA citizen 他是(个)美国人
His lucky charm is a penguin 他的幸运物是只企鹅
Now ，I think you shouldn't have trouble differentiate this :
If you use both affirmative 是 and 很, it just emphasize 很, nothing conclusive, but just agree others about something.
最近生意是很差 （Recently, the business is really bad )
(well, the business is really bad, last time one used to make $1000 a day, now only take home $990 a day, really bad.)
So in a conversation :
Lee : The business is bad
李 ： 生意很差
Business partner : Yupe, the business is bad， but it is not doom
生意伙伴 ：生意是很差, 不过还没那么坏
In short, you use 是 if agree on another person relative statement.
X : he is tall 他很高
Y : yes, he is really tall compare to his sibling, 是呀， 他是很高, 和他家的兄弟姐妹比较的话。
However, sometime you will prefer to use
really 真的 ， as agreement or emphasize your statement.
He is really tall , 他真的很高