In Mando'a, articles are used only if context does not make the importance of a noun clear. If what you want to express would be unclear without the article, use the word that best expresses what you want to say.
solus: one; one and only one instance of. Example: Ni copaani solus striil. I want one strill.
eyn: a, an; an instance of, any instance of, one of a number of, one of a few, any. This is the indefinite article. Use it when you want to emphasize that any of several similar options will yield the same outcome. Example: Ni copaani eyn striil. I want a strill. I want any strill.
te: the; a specific and known instance of, a previously mentioned instance of. This is the definite article, equivalent to most uses of "the" in English. Use this for when you want to refer to someone or something that you've just recently talked about in a different way. Use this to indicate that the thing you are talking about is a specific instance of something that meets requirements that your listeners know about and that no other instance of that something will work. Fans use te with the titles of Mand'alore (though using haar would be more accurate). The one canon example we have is from the original wordlist (jatnese be te jatnese, the best of the best; based on the idea that te is used for emphasis, this phrase implies that there are lesser versions of the larger "the best" group). Example: Ni copaani te alor. I want the alor, the one who heads up this group, but I don't know that person's name.
haar: the; a specific and "known by everyone" instance of; the unique instance of; the religiously, spiritually, or culturally important. This is the emphatic definite article. In English, we usually pronounce the emphatic version of "the" as "THEE." This is even more rarely used than te. The one example that we know uses haar (and not an unknown root) from the conlang's creator is Ke narir haar'ke'gyce rol'eta resol. The most well-known fan use is ibic haar yust. Example: Ni copaani haar mand'alor. I want THE Mand'alor (the real one, not any of the lesser ones or pretenders).
This Mando'a Lessons video presents information about including articles in Mando'a sentences: https://www.youtube.com/watch?v=SXUAS06MfwE