In http://pali.nibbanam.com/immerse.htm, it says
... adjectives in Pāli have to be singular when the noun is singular, plural when the noun is plural, and display by their form the same grammatical function as the noun they are modifying:
Potthakam mahantam hoti. (This is a large book.)
Potthake mahante honti. (These are has [some] large books.)
The great news about adjectives is that they all have the same endings as one of the noun declensions you will already have learned. You do have to learn that adjectives belong to different declensions, but their forms are ones you'll already know from having memorized the noun declensions earlier.
I don't get it why noun and adjective will have same ending unless an adjective without declension always has same ending with its nouns without declension?