Look at the sentence below please and tell me why the writer of this sentence made what looks to be a choice to modify the nouns with an indefinite article.

This essay contain a beginning paragraph, a middle paragraph and an ending paragraph.

Would it be wrong to modify them all with a definite article, like this? I feel very strongly and am pretty sure (Ha ha)  that there is only one beginning paragraph, one middle paragraph and one ending paragraph for this particular essay. 

This essay contain the beginning paragraph, the middle paragraph, and the ending paragraph.   

Could it help a little to read it this way?

This essay, just like many other short literary compositions, contains a beginning paragraph, a middle paragraph and an ending paragraph.

The indefinite is correct.

Think about this.

This box contains a crayon, a pencil and a piece of paper.

Would you always want to say the crayon, the pencil and the piece of paper? We would use 'the' only if they are a specific crayon, pencil and piece of paper we already know about and need to emphasise that it really is those particular items, and not just any crayon, pencil and piece of paper.

Thank you Nona and Jim.

The thrust?? of my inquiry lies with the article usage with such terms as "beginning" and "ending." Whenever I see a word preceded with such a term, instinctively I try to put the article "the" unless my close scrutiny shows it otherwise. I think my confusion is being further fueled by the fact that we normally associate a with the concept of one, in addition to the notion of it being an indefiite article, and when I see the sentence like the one below, it having a makes me concentrate too much on the "one" concept.    Help me to set what seems to be a misguided perception straight.

The essay contains a/the beginnig paragraph, a/the middle paragraph, and an/the ending paragraph. 

Let me tweak it a little and ask you this way. How is this sentence in this context different from the one above?

A teacher has a packet of papers to grade. Among the papers are the best paper, the second best paper and  the worst paper.

Here, even an item in the list is being introduced for the first time, it  is still being modified with the definite article the. What's your thought on this.  

Hi,

Let me try a different kind of explanation. The first time you mention something to me, say 'a'. That makes the thing specific/definite in our conversation. So thereafter, you can say 'the' and I'll know the thing you are talking about.

A man with a scar on his nose and a parrot sitting on his shoulder came into the room. The man sat down. The man smiled at me.

Best wishes, Clive