Definite articles are, well, definite. They point towards something directly. English has ONE definite article: "the."
Mag'ir'by is a little more difficult. Just a little. It has an additional article. The two articles are: "na" and "ne." Masculine and feminine. No neuter, here.
Let's start with some examples:
Ne shere - The car
Ni fra - The woman
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You probably aren't going to look here unless you haven't already read about the verbs. If you haven't go look the verbs up first.
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When one uses the pronouns e, i and o (he, she and it), "e" and "i" are used for masculine and feminine words, respectively, while "o" is only used for names. Now, how would you know what gender the named person or object is? Well, there are endings for that, too, but that will be covered in a different post.