El perro would be the dog.
A/an in Spanish is un, una, uno, unos, unas.
Don't write puerro, that means a leak.
Why is it "un perro" and not "el perro?"
Because it is "A dog", not "THE dog".
Because el means "the"
How do you when to use un or una
Una is for feminine words (Usually ending in "A").
Un is for masculine words
(Usually ending in "O").
You will learn more about this later. ¡Buena suerte!