Concerning the two 'the's, I agree. I'd also change 'human' to 'man'. I don't think 'nature' refers to human instincts here (although it could in principle). It just says 'nature (the environment) is stronger than man (with all technology)' - think of a natural disaster that destroys a city: humankind with all technology is helpless against the forces of nature.
In this case, "humanity" would also work although it is more commonly translated to "die Menschheit" in German. Note that "der Mensch" stresses more the nature of man (in the sense of Plato's ideas) than the group of all humans, therefore I feel 'man' is a better fit than 'humanity', but I may be wrong.
@soglio: http://youtu.be/pQiA8XymmKM A nice presentation on the topic of language and gender. It's in German and maybe not that easy to follow. Although I don't agree with him (I'm embracing militant feminism far less than he does), he raises some interesting and noteworthy points. The talk was held during a hacker event, so the audience and his choice of examples sometimes are a bit peculiar ;-)
Comparisons with wie and als are followed by a noun phrase in the same case as the thing you are comparing with.
Here, you are comparing der Mensch to die Natur, and die Natur is the subject of ist stärker and so is in the nominative case. Thus der Mensch is also in the nominative case.
Consider this pair of sentences:
Ich liebe dich mehr als mein Vater. "I love you more than my father".
(als) mein Vater is in the nominative case, so I must be comparing mein Vater to ich -- I love you more than my father loves you.
Ich liebe dich mehr als meinen Vater. "I love you more than my father."
(als) meinen Vater is in the accusative case, so I must be comparing meinen Vater to dich -- I love you more than I love my father.
this exercise uses "den mensch"
Huh? Where do you see the ungrammatical den mensch? Do you have a screenshot?
The sentence I see is Die Natur ist stärker als der Mensch with nominative der and with capital M on Mensch.
As for the case to use after als, please see my reply to Uberling.