Question 1

Why is it поставь and not ставь?

Accepted Answer

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.

Question 2

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

Accepted Answer

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.

Question 3

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

Accepted Answer

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.

Question 4

What case is будильник in?

Accepted Answer

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.

Question 5

Why does час become часов after восемь?

Accepted Answer

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.

Question 6

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

Accepted Answer

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.

Question 7

Why is there a comma before чтобы?

Accepted Answer

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 …

Question 8

What exactly does чтобы mean here?

Accepted Answer

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.

Question 9

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

Accepted Answer

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.

Question 10

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

Accepted Answer

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).

Question 11

Is Поставь formal or informal?

Accepted Answer

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.

Поставь будильник на восемь часов, чтобы не опоздать.

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