"My sister needs a pretty office."
Translation:Mi hermana necesita una oficina bonita.
A is feminine in Spanish meaning that you would say “Ese mujer es muy bonita.” O is masculine and example is “Ese hombre es muy bonito.” It wouldn’t make sense to use the feminine ending when you are talking to a male and same with the feminine ending. You could look up the website study Spanish for feminine and masculine endings. I hope this helped!
Some must come before the noun and some must come after. Some can be either before or after. Some are spelled differently when coming before than when they come after. Sometimes the meaning changes depending on if they come before or after. I think you just have to develop a personal relationship with each adjective.
Both are for the second person in singular form, but "necesita" si also for "she".