Here's a common rule If the definitive article shows gender like "das Kind" the adjectives occurring before the noun will have an 'e' ending. E.g "das schöne Kind" or "der grüne Vogel" otherwise it would have the 'er' and 'es' ending depending on the gender. E.g "das ist ein altes Buch" and "ein roter Teller"
The rule is in the link below
Thanks, I got it. It's weak inflection. For everyone else: here https://en.wikipedia.org/wiki/German_declension#Mixed_inflection.5B6.5D the difference between strong, weak and mixed inflection is explained well with declension tables.