I don't like reading and I got a lot of questions like this.
I did search "definite article" but have yet found a article that address this.
Let me give an example so those confused by my wording may know what I am talking about:
El alma, & las almas
I am not sure what is the reason for this though. It may be either that "la alma" sounds weird or that "alma" begins with a vowel.???
But yea, another example is words with specific endings, like "-cion"
Edit. There are many more examples like the ending "-dad".