Well, if you really want to get messed up, here's how it works with adjectives (Wade, 2010):

1: Singular Nominative.
одна высокая гора, один прекрасный день, одно окно


2-4: It depends.
mascul. & neuter nouns always go for Genitive Plural forms: "три родных языка"; "три больших окна" (that's a fixed rule)


Here comes the weirdness:
fem. nouns tend to take the Singular Plural forms: "три бедные сестры"

Exception 1 fem. noun switches ударение from Singular to Plural: "три высоких горы" (Sg.: гора; Pl: горы)
Exception 2 you are talking about fractionals/decimals "две/три/четыре целых"
Exception 3 you are talking about units in the sense of "по три спелых дыни"


Nouns that actually are adjectives "учёный" etc. stick to the regular rule when it comes to adjectives: 1 = Singular Plural; >2 = Plural Genitive

>4: Plural Genitive "5 малоизвестных фактов"; "10 прекрасных роз" (ужас);

In regular grammar books, they often talk about how to deal with nouns and don't care much about adjectives in the 2-4 region. At best, they talk about it when numbers are 5 or above. As you can see, there is a good reason why.