Here's why:
* Koko cute gorilla sign language difficult to learn: This is an opinion. While Koko was a famous gorilla who learned sign language, the statement about it being difficult is subjective.
* Everyone admires Hellen Keller: This is also an opinion. While many admire Hellen Keller, it's unlikely that *everyone* does.
* William Ellsworth Hoy was deaf: This is a verifiable fact. William Ellsworth Hoy was a famous baseball player who was deaf.
Let me know if you'd like more examples of fact vs. opinion!