brighter, brightest
bright, brighter, brightest
Comparative and Superlative forms are for Adjectives and brightly is an Adverb, therefore there is no comparative or superlative for brightly. The forms brighter and brightest exist as the comparative and superaltive forms of the adjective bright.
Consider the adjective "bright." The comparative form is brighter, the superlative form is brightest.
most brilliant
The comparative form of the word "ugly" is "uglier," and the superlative form is "ugliest."
What's the comparative and superlative of the word "exact"
The comparative is "stricter" and the superlative is "strictest".
Comparative: shallower Superlative: shallowest
The superlative of "sad" is "saddest" and the comparative is "sadder".
fewer - comparative & fewest - superlative
The superlative is cloudiest; the comparative is cloudier.
The comparative is later, and the superlative is latest