Пожалуйста, не роняй телефон на пол: экран уже один раз треснул.

This is a very common aspect question.

• роняй is the imperfective imperative of ронять.
• урони would be the perfective imperative of уронить.

In negative commands, Russian often uses the imperfective to mean:

• don’t do this in general
• be careful not to let this happen
• avoid this action

So Пожалуйста, не роняй телефон на пол sounds like a natural warning:
Please, don’t drop the phone on the floor.

If you said не урони, that would also be possible, but it can sound more like:

• make sure you do not drop it (even once)
• a more specific one-time warning

In everyday speech, не роняй is very natural here.

These are an aspect pair:

• ронять = to drop, to let fall — imperfective
• уронить = to drop, to let fall — perfective

The difference is not the basic meaning, but how the action is viewed:

• ронять focuses on the process, repeated action, or general idea
• уронить focuses on a single completed event

Compare:

• Он часто роняет телефон. = He often drops his phone.
• Он уронил телефон. = He dropped the phone.

So in your sentence:

• не роняй = don’t go dropping it / don’t let it fall
• треснул later in the sentence is also perfective, because it refers to one completed cracking event

Because Russian uses different cases depending on whether you mean movement toward a place or location in a place.

Here:

• на пол = onto the floor → motion toward a destination → accusative
• на полу = on the floor → location → prepositional

So:

• уронить телефон на пол = to drop the phone onto the floor
• телефон лежит на полу = the phone is lying on the floor

This is one of the most important Russian case patterns to learn:

• куда? (to where?) → often accusative
• где? (where?) → often prepositional

Because пол is an inanimate masculine noun, and for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: пол
• accusative: пол

You can still tell it is accusative from the grammar of the phrase:

• на пол after a verb of motion/falling means onto the floor

This is normal in Russian. For example:

• на стол = onto the table
• в дом = into the house

They may look unchanged, but they are functioning as accusative forms.

Russian does not have a direct tense equivalent to the English has cracked or has cracked once before.

Instead, Russian usually uses the past tense, often with words like уже to give the same idea.

So:

• экран уже один раз треснул

literally looks like:

• the screen already cracked one time

but in natural English it often means:

• the screen has already cracked once
• the screen already cracked once before

So this is completely normal Russian usage. English and Russian simply organize time differently here.

Because the sentence refers to one completed event: the screen cracked once.

• треснуть → perfective → to crack (once, as a completed event)
• трескаться / трещать suggest a process, repeated cracking, or the sound/ongoing action

Here the speaker means:

• the damage already happened once
• this is the reason to be careful now

So треснул is the right choice.

Compare:

• Экран треснул. = The screen cracked.
• Экран трескался. = The screen was cracking / used to crack / cracked repeatedly, depending on context.
• Экран трещал. = The screen was making a cracking sound / crackling, or was in the process of splitting

Один раз literally means one time or once.

So:

• Экран уже один раз треснул = The screen has already cracked once.

This adds an important nuance:

• it has happened before
• there is a reason to be extra careful

You could say just:

• экран уже треснул = the screen has already cracked

But один раз emphasizes that this happened once before, and implies:

• don’t let it happen again

It is a very natural phrase in Russian.

Уже usually means already, and here it modifies the whole idea that this happened before.

• уже один раз треснул = already cracked once

This word order is natural because уже comes before the time/occurrence expression.

Other orders are possible in some contexts, but this one sounds very normal and neutral.

For example:

• Он уже два раза звонил. = He has already called twice.
• Я уже один раз это видел. = I’ve already seen this once.

So the sentence’s word order is standard and idiomatic.

The colon links the two parts in a reason/explanation relationship.

• Пожалуйста, не роняй телефон на пол:
  экран уже один раз треснул.

The second part explains why the speaker is saying the first part.

So it is similar to English:

• Please don’t drop the phone on the floor: the screen has already cracked once.

You could also express this with a conjunction, for example:

• Пожалуйста, не роняй телефон на пол, потому что экран уже один раз треснул.

That means:

• Please don’t drop the phone on the floor, because the screen has already cracked once.

The colon is just a more compact written way to show that explanatory relationship.

Because пожалуйста here is being used as a polite introductory word, similar to please in English.

So Russian normally writes:

• Пожалуйста, не роняй телефон...

That comma is very common when пожалуйста introduces a request.

You may also see пожалуйста in other roles, for example:

• Спасибо. — Пожалуйста. = Thank you. — You’re welcome.

But in requests like this, the comma is standard.

Not exactly in the same way.

ронять / уронить

These mean to drop, to let fall, usually accidentally.

• уронить телефон = to drop the phone

бросать / бросить

These mean to throw.

• бросить телефон на пол = to throw the phone onto the floor

So if the idea is accidental dropping, ронять is the right verb.
If you used бросать, it would sound more forceful, as if someone is tossing or throwing the phone down.

Экран is a masculine noun, and past tense verbs in Russian agree with the subject in gender and number.

So:

• экран = masculine singular
• therefore the past tense form is треснул

Compare:

• экран треснул = the screen cracked
• дверь треснула = the door cracked
• стекло треснуло = the glass cracked
• экраны треснули = the screens cracked

This is a basic but very important Russian pattern: in the past tense, verbs show gender in the singular.

No, they are different places:

• на пол = onto the floor
• на землю = onto the ground

Use пол for an indoor floor, or a floor surface in a room/building.
Use земля for the ground, earth, soil, outside surface.

So in your sentence, на пол is correct because we are talking about dropping a phone onto a floor.