Which sentence states a fact?

 - The Canadian national anthem is difficult to sing.
 - "O Canada," the Canadian national anthem, was written in 1880.
The first sentence states a fact.
"O Canada," the Canadian national anthem, was written in 1880.
It can be proved by looking up when "O Canada" was written.
The second sentence states an opinion.
The Canadian national anthem is difficult to sing.
Difficult shows what a person believes, thinks, or feels. Another person might have a different opinion about what makes a song difficult to sing.