Quick answer
Hard: но́вый, но́вая, но́вое, но́вые. Soft: си́ний, си́няя, си́нее, си́ние. Every ending swaps its vowel for the soft equivalent.
Side by side
| Form | Hard: но́вый | Soft: си́ний |
|---|---|---|
| masculine | но́вый | си́ний |
| feminine | но́вая | си́няя |
| neuter | но́вое | си́нее |
| plural | но́вые | си́ние |
| genitive m. | но́вого | си́него |
| dative m. | но́вому | си́нему |
The mapping is mechanical: а→я, о→е, ы→и, у→ю. Nothing else changes, and the stress never moves.
The members of the club
Notice the pattern in the meanings: most of them are built from time words. If an adjective comes from a noun of time or place, it is a good bet that it is soft.
Why си́ний and not голубо́й
Си́ний is dark blue and soft; голубо́й is light blue and hard. Russian treats them as two separate colours, not two shades — which is why both appear in the basic colour vocabulary and why the sky is голубо́е while a navy suit is си́ний.
Not the same as the spelling rule
Adjectives like ру́сский or хоро́ший look soft in the masculine but are hard stems obeying the к, г, х, ж, ш, ч, щ spelling rule: хоро́шая, хоро́шее, not хоро́шяя.
Typical mistakes
си́няя не́бо
си́нее не́бо — or rather голубо́е не́бо
Neuter takes -ее, and the sky is light blue.
после́дныий
после́дний
A soft stem keeps -ний throughout.
Check yourself
What is the neuter of си́ний?