I read the following sentence:
Pourquoi la nature est-elle polluée ?
But I think the subject here is la nature and the verb is est, with polluée is an adjective.
So the following sentence should be fine:
Pourquoi est la nature polluée ?
But then why is it transformed into est-elle and what is the difference between the two sentences?
You can't construct a "why" sentence in French exactly like you do in English.
It's actually pretty similar if you look at it the right way:
Is nature polluted? = La nature est-elle polluée ?
Why [is nature polluted]? = Pourquoi [la nature est-elle polluée] ?
It works the same way with the other ways to ask a question in French:
Est-ce que la nature est polluée ? → Pourquoi [est-ce que la nature est polluée] ?
La nature est polluée ? → Pourquoi [la nature est polluée] ?
So a "why" question in French is just a normally constructed question with a "pourquoi" in front of it.
I found out that if the subject is NOT a pronoun, you must add the pronoun of the subject as well as the subject itself. In this case, since the la nature is the subject but not is a pronoun, you must add elle, a pronoun for the third-person singular (female).
So for example, if the subject is a pronoun, you can just invert the subject and verb:
Vous parlez italien.
Parlez-vous italien?
But in the following case, you need to add a pronoun:
La petite fille veut un vélo.
La petite fille veut-elle un vélo?
Source: https://www.dummies.com/languages/french/how-to-form-a-question-in-french-using-inversion/
