"Sarai sempre un buon amico."
Translation:You will always be a good friend.
This translation is a split infinitive in English. I wrote you will be always a good friend and Duolingo marked it wrong.
That is not a split infinitive. It's just a sequence of verbs, which allows you to place adverbs in between.
Would you ever say: "You can do always it", instead of "you can always do it"?