"We are young and pretty."
Translation:Jsme mladé a hezké.
Both 'a' and 'i' can be used for linking words together, 'i' is perceived as 'more strong' and you use it if you want to really emphasize the link.
Furthermore, there are some other situations where you use either 'a' or 'i', but I don't want to get into this. Too complicated for beginners.
Hope it helps and happy learning! :)