In the Russian language all nouns are assigned to genders (masculine, feminine and neuter). I don’t know exactly why, but that has happened and we have to deal with it. :) Masculine nouns typically end in a consonant or the soft sign: стол (table), вечер (evening), день (day, afternoon); there are several nouns ending in –a, –я that refer to men, such as папа (papa), дядя (uncle), дедушка (grandfather). Feminine nouns end in –a, –я or the soft sign: ручка (pen), семья (family), ночь (night), любовь (love). Most neuter nouns end in –o, –е: утро (morning), море (sea). There are several nouns ending in –a, –я which belong to neuter gender, such as имя (name), знамя (banner), время (time, tense), племя (tribe). Adjectives in Russian must agree with the noun in case, gender and number. Добрый – masculine, добрая – feminine, доброе – neuter.

Because morning is neuter, afternoon and evening are male gender.

I can explain: Доброе утро! - Good morning("it") Добрый день! - Good afternoon("he") Добрый вечер! - Good evening ("he") When say доброе it means that it refers to "it", when you say добрый it means that you refer to "he"

просто склонение. потому что утро - "оно", день - "он"...