Prepositions of duration and timing

How Russian distinguishes 'in two hours', 'for two hours', and 'two hours ago'.

English uses 'in' and 'for' loosely. Russian is precise — different prepositions and cases mark whether you mean duration, completion time, the moment of an event, or how long ago.

через + accusative — 'in / after'

How long until something happens. Через час = in an hour / an hour from now. Я приду через два дня. = I'll come in two days. The noun after через is in the accusative.

за + accusative — 'within / in (completed in this much time)'

Used with perfective verbs. Marks how long it took to complete the action. Я прочитал книгу за два часа. = I read the book in two hours (completed it within that time). за + accusative answers 'how quickly was it done?'

Bare accusative — 'for' (duration)

Just put the time period in the accusative. No preposition. Used with imperfective verbs to say how long something lasted. Я работал два часа. = I worked for two hours. Анна жила в Москве пять лет. = Anna lived in Moscow for five years. This is 'how long was it happening?'

назад — 'ago'

Time + назад = X ago. Два часа назад = two hours ago. Я приехал неделю назад. = I arrived a week ago. Note the accusative form (неделю, not неделя).

в + accusative — 'at (a moment)'

For specific time points: в три часа (at 3 o'clock), в среду (on Wednesday), в этот момент (at that moment). The noun is in the accusative.

Examples

Я приду через час.

I'll come in an hour.

Анна прочитала книгу за два часа.

Anna read the book in two hours.

Perfective + за.

Иван работал три дня.

Ivan worked for three days.

Bare accusative duration.

Мы приехали неделю назад.

We arrived a week ago.

Я позвоню тебе в семь часов.

I'll call you at seven.

в + accusative for clock time.

За пять минут я всё сделаю.

I'll get it all done in five minutes.

за + perfective.