If i had one complaint with Duolingo, its this. Youll notice that there is a list of words for each lesson, but often can complete lessons without all the words being introduced. Then you take the skill checks later and get blindsided. I never learned this word the first time through either.
There's lots of irregular gender nouns so it helps to always know the article (ex: a hand = "la mano"). Also in Italian the definite articles are used much more commonly then they are in English. For example in English we say "she eats apples" but in Italian we say "lei mangia le mele."