The last digit of a number decides the noun's form: 1 takes the plain dictionary (nominative singular) form, 2-4 take the genitive singular, and 5-9 plus 0 take the genitive plural. The count restarts with each new ten (21 behaves like 1, 22 like 2), but the teens 11-14 are an exception and always take the genitive plural.
Examples
- один час, два часа, пять часов
- one hour, two hours, five hours
- У меня три брата и шесть сестёр.
- I have three brothers and six sisters.
- Осталась двадцать одна минута.
- Twenty-one minutes remain.
- В группе двадцать два студента.
- There are twenty-two students in the group.
The mistake to avoid
Saying "пять часа" or "два часов" — and forgetting that 11-14 take the genitive plural (одиннадцать минут) even though 21 follows 1.
Where this fits in
Numbers, days, time, Russian A1: Count, talk about days and months, give the time, and handle numbers in real situations.