"She is not a child."
Translation:Ea nu este copil.
You use a different form of each verb depending on who's doing the action. This way, you can leave the subject out altogether and still be understood!
A FI - to be
So basically, ești = you are and este = he is / she is.
Yes - while there is some leeway (especially when writing for effect e.g. in prose, novels etc) generally word order does matter if you want to be technically correct. And with more complex sentences, not ordering your words correctly will either make the sentence nonsensical or just wrong.