I always think sopa sounds like soap. I got it right, but I don't think I'd like to drink soap soup.
In the music "Soap" by Melanie Martinez "Guess I better wash my mouth out with soap" I thought it was "Acho que é melhor eu lavar minha boca com sopa"(Guess I better wash my mouth out with soup)! Don't laugh at me.
it's interesting that in romance & Slavic languages our/us generally start with nas/nos or nash
Could we say "A nossa sopa"? Since there was one with "Os teus livros"?
Yes, it is also right.
What's the difference?
Nothing at all. When you have "possessive + noun", using the article is optional.