"He is not a girl, but a boy."
Translation:Ő nem lány, hanem fiú.
Hey! Well spotted. In the "Tips & Notes" section, there is a little explanation of when you would use van.
When you just say something like "She is smart" you do not use van in Hungarian, it's just "Ő okos."
You need van with adverbs like "here", "there", "inside", etc. and with questions words like "where", "how", etc., so "How is she?" is "Ő hogy van?".
It is technically wrong, it's impossible to use it here.
The reason is that when the subject is third person and you express a property of the subject, e.g. something like John is X., then you can't use van.
You can say Ő nem egy lány, hanem egy fiú. But van is impossible. I hope this helps!