Learn Dutch: Vervuiling is immers schadelijk voor de natuur. - Pollution is harmful to nature, after all.

Immers is a consensus adverb. It is used to justify a statement by pointing to a fact that the speaker assumes the listener already knows or agrees with. In English, it usually translates to "after all", "as you know", or "since". Here, the speaker assumes we all agree that pollution hurts nature.

Why is immers placed right after is?

As an adverb showing the speaker's perspective, immers belongs in the middle field of the sentence. In a standard main clause, this means it usually comes right after the main verb (is) and the subject. Since the subject here is the noun Vervuiling, immers drops in directly after the verb.

English just says "harmful to nature". Why does the Dutch sentence add de to make it de natuur?

In Dutch, broad concepts and generalizations often require a definite article (de or het), even when English leaves it out. When you talk about "nature" as a general whole, you must say de natuur.

Is the word schadelijk related to the Dutch word for damage?

Yes! Schadelijk is built from the noun schade (damage or harm) and the suffix -lijk (which works like the English suffixes "-ly" or "-ful"). So it translates directly to "harmful" or "damaging".