в + Prepositional (Time)

This construction is used for general timeframes like months, years, and centuries. It is not used for specific dates (day + month), seasons, or times of day.

• Months: All 12 Russian months are masculine and take the standard prepositional ending -е. Note that the spoken stress shifts to the ending for some months (like в январе) but stays on the stem for others (like в мае).
• Years: Years use the ordinal numeral form. The word год (year) uses a special Locative ending -у when used with в or на: в прошлом году (last year) or в две тысячи двадцать третьем году (in 2023). Note that the standard Prepositional ending -е is still used with the preposition о (e.g., о Новом годе). For compound years, note the rule for compound numerals: only the final number declines as an ordinal, while the preceding numbers remain cardinal and undeclined (e.g., в тысяча девятьсот девяносто пятом году).
• Decades: Use the plural ordinal numeral and the word годах: в девяностых годах (in the nineties). Native speakers frequently omit the word годах and simply say в девяностых.
• Centuries: Like years, centuries use ordinal numbers: в двадцатом веке.
• Ongoing Hours: The word час (hour) takes the Locative ending -у when referring to an ongoing hour: в каком часу? (at what time/during which hour?), в первом часу (just after twelve / in the first hour).
• Periods of Life: Use this pattern for life stages: в детстве (in childhood), в молодости (in youth). However, for specific ages, use в + Accusative (e.g., в двадцать лет).
• Demonstrative Pronouns: To say "this" or "that" for the time periods covered by this pattern (months, years, centuries), use the prepositional forms of demonstrative pronouns: в этом месяце (this month), в этом году (this year), в том месяце (that month), в том году (that year).
• Fixed Expressions: This pattern is also used for parts of a timeframe: в начале (in the beginning), в середине (in the middle), в конце (in the end), в первой/второй половине (in the first/second half), and for general time concepts like в прошлом (in the past) or в будущем (in the future).

Important Contrast:
1. The word неделя (week) is an exception; it uses на instead of в (e.g., на следующей неделе).
2. Do not use this for seasons or times of day (which use the Instrumental, e.g., утром, вечером) or days of the week (which use the Accusative).
3. Clock times also use the preposition в, but they take the Accusative case, not Prepositional (e.g., в три часа - at three o'clock).
4. To express 'in [time period]' meaning 'after a period of time', use через + Accusative (e.g., через несколько недель - in a few weeks).
5. The words время (time) and раз (time/occurrence) take the Accusative case with в, not Prepositional (e.g., в это время - at this time, в следующий раз - next time). Do not say *в этом времени.

Phonological Note: The preposition в changes to во if the following word starts with a difficult consonant cluster, particularly those starting with в or ф followed by another consonant (e.g., во втором тысячелетии).