"You are a boy."
Translation:Tu es puer.
As far as I know, either is OK and even "Puer es" (without the pronoun).
I learned in a discussion yesterday that the version ending in the verb is the more usual one.
I guess that the contributors chose the word order that is closer to English to make it easier for us at this point.
Statistics favour "Puer es", then "tu puer es" to emphasize "you and not others", then "tu es puer" or "es puer" a very strange and unusual word order to emphasize that "you exist as a boy, and you would not be neither a man nor a girl".
Because "tu" is the subject of the sentence (you are). Therefore, in Latin it has to be in the nominative case - tu.
"Tibi" is a different case, dative, expressing "to you". "Gratias tibi ago" means "I give you thanks", and here, "tibi" is the dative case of the pronoun "tu" because this person is the recipient of the thanks.
Here you can see the pronoun "tu" with its different cases: http://latindictionary.wikidot.com/pronoun:tu