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.