Questions & Answers about Раньше в клубе были одни мужчины.
Why use the plural form одни if it means only?
Одни is the plural form of the word один. When used directly before a plural noun like мужчины, it acts as a restrictive particle meaning only or nothing but. It behaves like an adjective, agreeing with the noun in both number and case.
Could I use только instead of одни in this sentence?
Yes, you could say Раньше в клубе были только мужчины. Both are correct and natural. However, using одни gives a strong conversational flavor of exclusively or nothing but, emphasizing the total absence of anyone else.
Why are одни and мужчины in the Nominative case rather than the Genitive?
You might be thinking of adverbs of quantity like много, which force the following noun into the Genitive case. But одни acts like an adjective and simply matches its noun. Since the men are the subject of the sentence, мужчины is in the Nominative case, and одни copies it.
Does мужчина decline like a feminine noun even though it refers to men?
Exactly. Мужчина is grammatically masculine but ends in -а, so it uses the same case endings as feminine nouns. That is why its plural is мужчины. Any adjectives attached to it, however, still take normal masculine or plural endings.