Hello, Kelsey! These words have the different declension: The words with the endings: "ИЯ", "ИЙ","ИЕ" have the ending "ИИ" IN PREPOSITIONAL CASE. ЛЕКЦИЯ(f) - НА ЛЕКЦИИ ГЕРБАРИЙ(m) - О ГЕРБАРИИ ЗДАНИЕ(n) -В ЗДАНИИ ОБЩЕЖИТИЕ(n) - В ОБЩЕЖИТИИ РЕШЕНИЕ(n) - О РЕШЕНИИ So, that was a good question!

Евгений is right. It happens only where one would expect И+Е and only in Dative and Prepositional. Nominative/Accusative retain -ие. -ИЯ (note dative): Мария (Maria), Марии, МарИИ, Марию, Марией, о МарИИ. Cf.: Ваня, Вани, Ване, Ваню, Ваней, о Ване. братия, братии, братИИ, братию, братией, о братИИ. (a collective noun, fraternity [of monks]) -ИЙ Юрий, Юрия, Юрию, Юрия, Юрием, о ЮрИИ. (There nouns are mostly foreign: нация (nation), коронация (coronation) - those with -ion/-ia in English. Амфибрахий, Гай Юлий Цезарь. Some like братия are slavic collective nouns) Also neuter nouns on -ие: Nom./Асс. пониманИЕ, Prep. о пониманИИ These are mostly (some exceptions around) abstract nouns similar to English -ing gerunds: even здание=buildING. PS. You must have noticed that they have two spellings: терпение and терпенье - both occur in books. 'patience'. Наталья and Наталия - both occur in IDs. And even the 'folkish' variant of Maria: Марья. Unlike терпенье, most of 'long abstract nouns' occur in shortened -ье form in poetry, but in full -ие from in books. Due to rythmic considerations I suppose, but both forms belong to the language. And the opposite: some shorter and less abstract -ье nouns have an archaic -ие parallel variant: счастие instead of счастье. Normally -ИИ prepositional is chosen for 'full' -ие forms, and -ЬЕ for reduced. But don't be surprised to meet Prep. "платьи":-)

Russian grammar is very complicating. In order to learn the rules and the way the word ending changes we have to study morphological rules including noun cases. Above people gave clear answer why and how word endings change.