|You already know 'un' is the Spanish undefinite article, masculine, which corresponds to the English "a". We also have the word "uno", which is an undefinite pronoun.|
So, we can either say: "Quiero leer un libro interesante" or "Quiero leer uno interesante".
As you can see, "uno" replaces the word "libro".
The difference between both is that "un" is always followed by a noun (in this case, "libro"), but "uno" replaces a noun.
So, from this point, we can undestand that a sentece such as: "Yo quiero uno libro" is not correct. Let's see some examples:
Hope this helps!
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