I know that for short adjectives you can add 'er' for comparatives, and 'est' for superlatives. But what about 'tired'? 'Tireder' doesn't exist, does it? Same for 'tiredest'
I am the tiredest man in the world....
Tired very rarely takes -er or -est to form its comparative and superlative degrees. *Tireder (which occurs only 5 times in BNC [British National Corpus]) or *tiredest (which occurs only 1 time in BNC) might be regarded as unacceptable or ungrammatical. Most tired is not often seen; more tired is a bit common.