When you encounter an abbreviation like США (USA) or ООН (UN), how do you know which gender to use for past-tense verbs and adjectives? Since most of these are indeclinable (their endings don't change), you can't just look at the last letter.
The Rule of the Core Noun
The trick is to identify the core noun in the full, unabbreviated phrase. The abbreviation automatically inherits the gender and number of that main word.
Plural Acronyms
США stands for Соединённые Штаты Америки (United States of America). Because the core noun штаты (states) is plural, США takes plural verbs and adjectives.
Similarly, СМИ stands for Средства массовой информации (Mass media). The core noun средства (means) is plural.
Feminine Acronyms
If the core noun is feminine, the abbreviation takes feminine modifiers and past-tense verbs (ending in -ла).
- ООН = Организация Объединённых Наций (UN). Core noun: организация (organization).
- РФ = Российская Федерация (RF). Core noun: федерация (federation).
- ФСБ = Федеральная служба безопасности (FSB). Core noun: служба (service).
Masculine Acronyms
Masculine core nouns make the abbreviation masculine, meaning past-tense verbs end in -л.
- ЕС = Европейский Союз (EU). Core noun: союз (union).
- МГУ = Московский государственный университет (MSU). Core noun: университет (university).
Neuter Acronyms
If the core noun is neuter, the abbreviation is neuter, and past-tense verbs end in -ло.
- ДТП = Дорожно-транспортное происшествие (traffic accident). Core noun: происшествие (incident).
- СНГ = Содружество Независимых Государств (CIS). Core noun: содружество (commonwealth).
- МЧС = Министерство чрезвычайных ситуаций (Ministry of Emergency Situations). Core noun: министерство (ministry).