Which sentence states a fact?

 - Jackie Robinson is the greatest baseball player in modern major league baseball history.
 - Jackie Robinson was the first African American baseball player in modern major league baseball.
The first sentence states a fact.
Jackie Robinson was the first African American baseball player in modern major league baseball.
It can be proved by reading a book about the history of baseball.
The second sentence states an opinion.
Jackie Robinson is the greatest baseball player in modern major league baseball history.
Greatest shows what a person believes, thinks, or feels. Another person might have a different opinion about what makes a baseball player great.