An adjective (विशेषण, viśeṣaṇa — "that which specifies") describes a noun. The rule: the adjective agrees with its noun in (1) gender, (2) number, (3) case. Example with नव (nava = new): "the new boy" = नवः बालकः (both masculine nominative singular); "the new girl" = नवा बालिका (both feminine nominative singular); "the new fruit" = नवम् फलम् (both neuter nominative singular). Adjective word order is flexible — नवः बालकः and बालकः नवः are equally correct. The endings tell you what agrees with what, not the position.