Why can't I say "Bonan tago" ? What's the difference between tago and tagon ?
Both words are in the accusative (bona tago is the nominative). You can read more about that in the Tips and Notes section: https://www.duolingo.com/skill/eo/Common-Phrases / https://www.duolingo.com/skill/eo/Accusative
Hey look! Deutsch!
Really, it's a weird mix of a latinida lingvaro adjective and a ĝermana lingvaro noun. Classic Esperanto.
I was wrong in saying bone tago? Whats the difference?
You can find more information about why greetings are written using the Accusative if you click on the links I left in my previous post.
la portugala tradukto estas difekta! Atentu