"Hello, I am the man."
Translation:Saluton, mi estas la viro.
They mean the same thing, but we put an -n on the end of a word when it's a direct object.
Ŝi batas la viron = She hits the man. "She" is the subject of the sentence, "hits" is the action the subject is performing, "the man" is the object she is performing the action on.
La viro batas ŝin = The man hits her. "She' is now the object of the sentence. See how the -n on a different word changes the meaning of the sentence?
(You can even change the word order. "Ŝin batas la viro" still means "The man hits her." The -n tells us which is the direct object, no matter the order of the words in the sentence).
There is one exception: -n is never used with the verb estas. (That's why this sentence is "Mi estas la viro" and not "Mi estas la viron"). That is because to be something is not considered an action, but a state of being.
We call this -n the accusative. You can find a more thorough explanation of it in the Tips & Notes of the Accusative lesson.