You use "e" when it comes before a word that starts with the "i" sound. To see why, try saying "hijos y hijas" quickly. You'll notice that you can't hear the "y" and it sounds like "hijos hijas." Change the "y" to "e" and it's more clear you're saying "hijos e hijas." AND y and o change if they are followed by the same vowel sounds. (basically if the word after y or o has the same sound then the word will change)
E.g ¿Vas a comprar siete cervezas u ocho? "Are you going to buy seven beers or eight?"
E.g Julieta e Ignacio estudian la medicina. "Julieta and Ignacio study medicine."
hope this helps!!
