Numbers ending with 2-4 (except for 12-14) put the noun in singular genitive case in Russian. 5-10, on the other hand, require a plural noun (also in genitive).

Numbers ending with 2-4 (except for 12-14) put the noun in singular genitive case in Russian. 5-10, on the other hand, require a plural noun (also in genitive). Since plurality is not regular and plural cases are even less so, some people will have trouble with deriving the correct plural case for some words. Something similar to some English speakers sometimes being confused about plural for Greek and Latin origin words ("forums"/"fora", "viruses"/"virii" etc.).