I'm not sure what the tips say, because they're different between the website and the apps and I'm not even sure what unit this appeared in, but the actual "rule" is that "buono" behaves like "uno":
- uno stato -> un buono stato
- un uomo -> un buon uomo
- un cane -> un buon cane
While the feminine "buona" is never elided in modern Italian, except in some old locutions:
- una casa -> una buona casa
- un'ora -> una buona ora BUT "alla buon'ora"
This link has some info: http://www.italyworldclub.com/learn-italian/course/grammar/adjectives.htm Unfortunately the list of adjectives changing meaning with position isn't complete: for instance "un alto ufficiale" is a high official, "un ufficiale alto" a tall official.