"The girls like books."
Translation:הילדות אוהבות ספרים.
Only when a word is an adjective that describes the girls or a verb that is referring to them, would it change to match. Books has nothing to do with the girls. It is a noun all of it's own and does not change to match the girls. "אוהבות" changes because it is a verb and is referring to the girls. If you were saying the girls are pretty you would say "הילדות יפות." and "יפות" would change to match girls because it is an adjective that is describing what kind of girls they are.
Does that make sense? I am happy to help. Let me know if that makes it more clear or if that just complicates it more. :)