The main difference between "un" and "una" is that un is masculine and una is feminine so you have to use un or una depending on the word, if the word is masculine like "ordenador", "anillo" "amigo" you have to use un, but if the word is feminine like "letra" "amiga" "profesora" you have to use una, so you fave to focus on the word below. I think it's important remind you when to use un/una in a sentence instead of el/la: We use el/la to refer to something specifically: "El perro de mi tía" you are talking about one dog specifically, your aunt's dog. However, we use un/una to talk about something not specifically. Look at this example: -¿Qué quieres beber? -Un café. In this case we would say "un café" cause we want a coffe, we don't mind which coffe, it' would be stupid to ask for a coffe specifically like "give me this coffe" you want a coffe and that's it I hope you have understood it with my silly examples hahaha but I think that jonking it's a goos way to learn.

un (masc.) o una (fem) acompañaran a un nombre o sustantivo. la diferencia con el o la es que estos últimos determinan al nombre (tu sabes de que te hablan Ej: el perro. Tu sabes que perro es del que te hablan) En cambio con un o una puede ser cualquiera. (Ej:un perro. Tu no sabes cual y tampoco importa.

un is for masculine nouns and una for femenine un lapiz, un perro, un calcetine unos lapices, unos perros, unos calcetines una casa, una mesa, una manzana unas casas, unas mesas, unas manzanas

In Spanish "un" and "una" is use like in English "a" or "an" except that we do not follow the exact same rules. What I mean is using "a" or "an" in English depends on whether the noun starts with vowel or consonant. However, in Spanish since we have grammatical genders, we use "un" for a masculine noun, and "una" for a feminine noun. For exemple: Una computadora (Computadora is a feminine noun, which means computer); un refrigerador (Refrigerador is a masculine noun which means refrigerator).