The comparative and superlative forms of adjectives can be formed in four different ways.

1. Usually comparative and superlative forms are formed by adding the inflectional endings -are and -ast to the end of the word.

Vad är den trendigaste hårfärgen just nu?

What is the most fashionable hair colour right now?

2. With some adjectives, the comparative and superlative endings appear in the shorter forms -re and -st. Most often, the vowel of the adjective stem changes at the same time.

Trots att priset var högre var kvaliteten sämre.

Despite the higher price, the quality was worse.

3. There is also a small group of adjectives whose comparative endings are the previously mentioned -re and -st, but the stem of the adjective changes completely:

Camilla är en bättre läkare än Martina.

Camilla is a better doctor than Martina.

4. The comparative and superlative of adjectives ending in -isk and participles (e.g. the endings –ande, –ende and –ad) are formed with the help of the words mera and mest.

Den där affären har det mest omfattande urvalet av produkter.

That store has the most comprehensive selection of products.

Superlatives have two forms, short and long. The short form is used when it is not followed by a noun or a sentence starting with som. The short form is not inflected:

Dessa tröjor är bekvämast.

The long form can be used both before a noun (min bekvämaste tröja) and independently, in which case you should remember the definite article den/det/de before the superlative: 

Denna tröja är den bekvämaste.

If the superlative is followed by a sentence starting with som, the long form is used.