Superlative adjectives are used to compare more than two objects and show the greatest degree. They are formed by adding "-est" to regular adjectives like "big" becoming "biggest" or changing "y" to "i" and adding "-est" like "pretty" becoming "prettiest". Examples show Jupiter as the biggest planet, the smartest girl in class, and Mount Everest as the highest mountain in the world. Irregular adjectives also have superlative forms.