"You are not the boy."
Translation:Vi ne estas la knabo.
Umm you have to put the negative indicator before the verb.. "Ne estas"
Ne ludas (do/ does not play) Ne mangxas(do/does not eat)
"La" always means "the" and never "a" the article "a" is implied when a noun stands alone with out any other article or quantifier. For instance: Knabo = a boy, (any boy, not THE boy, not two boys, not three boys, etc...) La knabo = the boy (you are refering to a specific boy, not any boy)