Ignacio: Just for conversation's sake, here are the rules of accentuation:
If the word ends in "n", "s" or a vowel, the word is naturally stressed on the penultimate syllable.
If the word ends in any other consonant, the word is naturally stressed on the last syllable.
Exceptions to these two rules are marked with an orthographic accent.
That's about the simplest explanation you can give/get. Keep in mind, (as I'm sure you're aware)
there are volumes upon volumes written about this somewhat simple subject...
Isn't grammar exciting?