Briefly explain how we turn spanish words in their singlar form that end in an
"O" or an "a" into their plural form.

With adding a “s” at the end, for example with the word “sierra” if I wanted to make it plural I would just add a “s” at the end and it makes it “sierras” which is plural and also another is like with “carro” if I want to turn it into plural I’ll just simply add the “s” at the end and now it’s “carros”.