A general question rather than a specific one, though this is an example. The lessons, as I understand them, teach that short, and some common adjectives go before the nouns, but otherwise (unless for particular stress) most adjectives go after the noun.
However, I have noticed that often these rules don't seem to apply. Puissant is neither short (in my mind one syllable), or common. However the text above places it before, but after is acceptable as well when the exercise is marked.
I have noticed this many times in doing the exercises. As a consequence, I am confused.
If the simple answer is that "short" means 2 syllables, I will be content.