This is a really good question! You won't find "нь" in dictionaries because it's not really a word by itself, but kind of like a suffix. The most common usage of "нь" is third person possessive. As in, "my life", "your life", "her life" would be "амьдрал минь", "амьдрал тань/чинь", "амьдрал нь". In order to imply possession, you can use both possessive pronoun (1 миний, 2 таний/чиний, 3 тэрний) and possessive suffix (1 минь, 2 тань/чинь, 3 нь). For example, "тэрний амьдрал нь". You can also use only one of them and drop the other: "тэрний амьдрал" or "амьдрал нь". At the end of a sentence, we usually don't use suffix (so we have to use the pronoun). For example, "энэ бол миний амьдрал". Anyways, back to your question. "Аварга могойг гадна дотно талаас нь зур" literally translates to "draw the giant snake from its inside and outside." Here, "нь" corresponds to "its (snake's)." I think a proper English translation of the sentence (rather than literal translation) is "draw the inside and outside of the giant snake." It's just the way Mongolians speak. The second occurrence of "нь" shows another usage of "нь". What it does is it takes a sentence (газарзүй, түүх, тоо бодлого, хэлний зүйг сонирхон оролд - have interest in and attempt to learn geography, history, ...) and makes it the subject of another sentence (... оролдсон нь дээр - attempting to learn ... is better). Third occurrence is the same as the 2nd. "... нь итгэлийг минь алдагдуулж билээ." It says something discouraged him/her. That *something* is the sentence before "нь" (which is the fact that her "first and second drawings were unsuccessful"). Fourth one is also doing the same thing as the 2nd. "Explaining to adults is very boring."