"She is not a girl."
Translation:Ela não é uma menina.
é is the 3rd person, present tense, indicative form of the verb "ser", meaning "to be". When negating a verb the "não" comes first. So "it is" = "é" and "it is not" = "não é" (or "is" = "é" and "is not" = "não é"). Another example "I smile" = "eu sorrio" and "I do not smile" = "eu não sorrio". When you think about it, it is actually English with the inconsistency, since we usually put the negative before the verb or adjective, eg "no running", "not true", "never happened" but then say "it is not" and "do not" which have the negative last. There is not always a literal translation, you just have to learn the rules as you go on. What would make it hard is if the rules have many exceptions, or were complicated, but they are generally well behaved :)
Have a look at http://www.learn-portuguese-with-rafa.com/ because although more fun, I don't think Duolingo should be used alone to learn Portuguese. In my opinion it doesn't give a deep understanding of the language, and is not the best for learning how to construct Portuguese sentences.