Поставь будильник на восемь часов, чтобы не опоздать.

Set the alarm for eight o’clock so you won’t be late.

Breakdown of Поставь будильник на восемь часов, чтобы не опоздать.

• не: not
• опоздать: to be late
• час: the hour
• поставить: to set
• будильник: the alarm clock
• восемь: eight
• чтобы: so that / in order to
• на: on/for

Questions & Answers about Поставь будильник на восемь часов, чтобы не опоздать.

Why is it поставь and not ставь?

Поставь is the perfective imperative of поставить and suggests a single, completed action: set the alarm (once) to a specific time.
Ставь (imperfective) is more like be setting / set (as a process), set it regularly, or can sound like repeated instructions depending on context.

Does поставь будильник mean set an alarm or put an alarm clock somewhere?

It can mean either:

• set the alarm clock (very common meaning)
• put/place the alarm clock somewhere (literal physical placement)

In this sentence, на восемь часов makes it clearly mean set it for 8 o’clock.

Why is it на восемь часов and not в восемь часов?

Because with alarms Russian typically uses поставить/завести будильник на + time = set the alarm for + time.
В восемь часов usually describes when something happens: В восемь часов я встаю = At eight I get up.

What case is будильник in?

It’s the direct object of the verb, so it’s accusative singular. Since будильник is inanimate masculine, its accusative form looks the same as nominative: будильник.

Why does час become часов after восемь?

After numerals 5 and above (пять, шесть, семь, восемь…), Russian uses genitive plural:

• восемь часов (literally “eight of hours”)

So часов is genitive plural of час.

Is восемь часов here “eight hours” (duration) or “eight o’clock” (time)?

Here it means eight o’clock (a clock time), because it’s used with setting an alarm: на восемь часов = for 8:00.
The same form восемь часов can also mean eight hours (duration), but context usually makes it clear.

Why is there a comma before чтобы?

Because чтобы не опоздать is a purpose clause (a subordinate clause expressing purpose), and Russian normally separates it with a comma: …, чтобы … = …, in order to … / so that …

What exactly does чтобы mean here?

Чтобы introduces a purpose: in order to / so that.
So the sentence means: Set an alarm for 8 o’clock so that you don’t end up being late.

Why is it чтобы не опоздать (infinitive) and not чтобы ты не опоздал?

After an imperative, Russian often uses чтобы + infinitive when the subject is understood to be the same person:

• Поставь будильник, чтобы не опоздать = Set an alarm so as not to be late

You can also say чтобы ты не опоздал, which is more explicit and can sound a bit more emphatic.

Why is it не опоздать (perfective) and not не опаздывать (imperfective)?

Опоздать (perfective) = to be late (once / in a specific instance), to end up late. That fits this situation (one morning/one event).
Опаздывать (imperfective) = to be late repeatedly / habitually / as a general tendency.

So чтобы не опоздать is like so you won’t be late (this time).

Is Поставь formal or informal?

Поставь is the ты-form imperative (informal, used with friends, family, children, or peers).
A polite/formal version would be Поставьте будильник на восемь часов, чтобы не опоздать.