Thank you for your help. I should have come back sooner. Right after I asked I got a host of sentences and of course it is exactly as you say. I also noted that the pronunciation is altered with emphasis on the "e" (with accent). (Btw change the "thing" to "think". I do that all the time.)
Italian makes a strong distinction between singular'you' and plural 'you'. It has two verb forms for them:
(tu) sei -> 'you are' when addressing one individual.
(voi) siete -> 'you are' when addressing a group, like in 'you all' or 'y'all'.
* tu siete un ragazzo is incorrect because the subject (tu) doesn't match the verb form (siete is the plural form of you).
If you omit the subject pronoun, like in siete un ragazzo then you're saying 'you all are a boy'.