oneThis plural form is used for numbers like: 1
In %1$d month
otherThis plural form is used for numbers like: everything else
In %1$d months
В %1$d месяц
В %1$d месяца
В %1$d месяцев
In %1$d month
In %1$d months
In %1$d month
In %1$d months
В %1$d месяц
В %1$d месяца
В %1$d месяцев
%1$d - number of months, %1$d may be skipped for singular if translation looks better.