Have I used "retard" word in a correct way?
"The main obstacle that retards nature conservation moves is human(or humanity?) itself"

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"The main obstacle that prevents nature conservation is humanity itself"

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This is the correct usage of the word if you want to communicate that it slows down or delays conservation. I would replace it with 'hinders', 'impedes' or 'hampers' if you want to communicate that it obstructs conservation.

Humanity is correct.