Cuantificadores - Artículos y cuantificadores negativos

used when we want to talk about a person or thing for the first time or when other people may not know who or what they are

un

used before a singular noun that starts with a vowel sound, when we are not talking about a specific person or thing

un

used when referring to a person or thing that was previously mentioned or one that is identified easily

el

used to show that there is none or almost none of something

ninguno

not any of the members of a group of people or things

ninguno

one or some of a thing or number of things, however large or small

cualquier

used to indicate the absence or lack of something

cero