"The girl reads a good newspaper."
Translation:La fille lit un bon journal.
I don't understand. Why in some cases adjective is before the noun and in some case after?
Some adjectives go before, others go after. Check here: http://french.about.com/od/grammar/a/adjectives_4.htm
There is a good rule of thumb B.A.G.S. which is Beauty, Age Goodness and Size. If an adjective goes into one of these categories it goes before the noun, otherwise it typically goes after the noun. "Bon" is a measure of how good something is so it goes before the noun.
and how did duolingo expect me to know this rule without informing me first?!
Good learning comes with frustration! It gets planted into your brain that it was frustrating, therefore making it remember not to make the same mistake twice.
Same reason you say "She reads" instead of "She read": it's just different conjugations of the verb lire (to read).
Why is the 'bon' masculine not feminine? I thought a girl was reading it??
Bon describes the newspaper, not the girl. In this case, "journal" is a masculine noun. So you must use the masculine adjective, which is "bon". For example:
Bon ami = good friend (masculine noun AND adjective)
Bonne amie = good friend (feminine noun AND adjective)