"He is not a girl, she is not a boy."
Translation:El nu este o fată, ea nu e un băiat.
E and este are two different ways to say the same thing: (he/she/it) is. There is no difference. As for whether Romanian is the hardest romance language, I think it depends on where you are coming from. From other romance languages or English, probably. But if you already know a Slavic language and have little experience with Spanish, French, etc., that would probably make Romanian a lot easier than the others.
There is no difference in meaning or grammar rules between El/ea este and El/ea e.
El/ea este is considered to be more formal.
El/ea e is used in spoken language.
El nu este o fată, ea nu este un băiat.
El nu e o fată, ea nu e un băiat.
nu - is the negation
El nu este o fată, ea nu e un băiat (they mixed the two forms, I do not know why...maybe just to show that there is no difference between them)