According to this: http://www.locallingo.com/czech/grammar/adjectives.html
(and the Tips & Notes in Masculine, too)

Czech adjectives are divided into two groups:

1) Adjectives with a hard ending in the nominative singular -ý (masculine)
-á (feminine)
-é (neuter)

2) Adjectives with a soft ending in the nominative singular
-í (masculine, feminine and neuter)

All Czech adjectives end with a long vowel.

It seems n is considered a soft sound.