"You are a nice boy."
Translation:Olet mukava poika.
"Te olette" is "you are" as in 2nd person plural "you".
"Te olette hauskoja lapsia" - "You are funny children"
You can also you "te" to indicate a single person though, if you want to be very polite and respectful (like in many other languages such as French, German, Russian, Spanish...). This is called "teitittely".
"Mukava" means "nice", but also "comfy/comfortable" as in "tämä tuoli on mukava istua" (this chair is comfy/nice to sit on), "tässä tuolissa on mukava istua" (it is nice/comfy to sit in this chair), "tämä tuoli on mukavan tuntuinen" (this chair feels comfy/nice), "tuntuu mukavalta olla tässä" (it feels nice/(comfy) to be here) etc.
"Kiva" also means "nice", but it does not mean "comfy". Instead, there is an aspect of fun in it.
"Meillä oli mukavaa" (We had a nice (time))
"Meillä oli kivaa" (We had fun)
When it comes to describing people these two words a quite interchangeable.
"Hän on mukava" (She/he is nice")
"Hän on kiva (He/she is nice)
I'd say "kiva" is more spoken than written Finnish though. If an author were describing a character that is nice, they'd be more likely to use "mukava". But this is just my two cents, perhaps someone has a different interpratation. :)
I inadvertently typed here 'olet kiltti poika' and duolingo accepted the word 'kiltti' (I recently learn this word here so it was playing in my mind).. Earlier few lessons I took, duolingo was very strict which words can I use and which I can't, but it seems now they have added some synonyms in the mix or at least the synonymous words we may have encountered in earlier chapters of the course. Nice!
"Olet poika mukava" emphasises the fact that you are addressing some boy and places more stress on that YOU specifically are mukava. It is completely normal Finnish even outside poetry. In actuality it reads as "sinä olet, poika, mukava". It is the same as in English: "you, lad, are grand" vs "you are a grand lad".