I'm not sure that you can, at least not directly. In English, "the better" really means the best of exactly two choices, whereas Spanish doesn't always make that distinction. (For example, entre means both "between" two things and "among" three or more.)
So you would either have to recast the sentence to say that Nature is a better teacher than the other thing you have in mind (Experience? Nurture? University?) or is the best teacher of the two. Either way, that which you're comparing Nature to would have to be stated directly, or already established within the context of the conversation.
Of course, I could also be way off. Hopefully a native speaker will see this at some point, and either confirm or refute this explanation.
Most adjectives come after the subject, however there are quite a few that generally go before the subject (mejor / peor), and quite a few that change the meaning of the sentence depending on whether they are before or after the subject.
Here are some examples:
- La chica pobre (The poor girl that does not have money) / La pobre chica (That poor pitiful girl)
- Mi amigo viejo (My old friend, he's like 100 years old) / Mi viejo amigo (My long-time friend)
More information here: http://spanish.about.com/cs/grammar/a/whereadjective.htm
Apart from what amoussa said, as Spanish don't have comparative and superlative form (at least that's what I learnt), mejor comes before the noun helps showing these things.
maestro mejor = good teacher
mejor maestro = better teacher
LA mejor maestro = THE best teacher
Hope this helps :)
I don't know for sure about maestro mejor but your assertion that Spanish does not have comparative form is self-evidently false. Mejor is to bueno as better is to good! You are correct that putting the article in front makes the suprrlative - el mejor = the best. As with longer adjectives in English (shorter ones mostly simply adding -er e.g. shortER) most comparatives are formed by inserting 'more' in Spanish usually insert 'más' before the adjective.