После удара экран телефона треснул.

Because после requires the genitive case.

• dictionary form: удар
• genitive singular: удара

So после удара literally means after the impact / after the blow.

This is a very common pattern:

• после урока — after the lesson
• после работы — after work
• после дождя — after the rain

This is the normal Russian way to say the phone’s screen or the screen of the phone.

Russian often shows possession with:

• the thing possessed in the base form
• the owner in the genitive

So:

• экран — screen
• телефон — phone
• телефона — of the phone

Together, экран телефона = the phone’s screen.

This is much more natural in Russian than trying to copy the English ’s pattern.

Because телефона is the genitive singular form of телефон.

Here it is used after экран to show possession:

• экран телефона — screen of the phone

This same pattern appears everywhere in Russian:

• дверь машины — the car door
• центр города — the city center
• имя брата — the brother’s name

So the ending -а here is not random; it marks the genitive singular of a masculine noun.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is экран, which is:

• singular
• masculine

So the verb must be masculine singular:

• треснул

Compare:

• экран треснул — the screen cracked
• дверь треснула — the door cracked
• окно треснуло — the window cracked
• стёкла треснули — the panes cracked

So the ending tells you about the subject.

The subject is экран.

A useful way to see it:

• После удара — time/circumstance phrase
• экран телефона — subject noun phrase
• треснул — verb

Even though телефона comes right after экран, it is not the subject. It only modifies экран and means of the phone.

So the sentence is built around экран ... треснул.

In this sentence, треснул means that the screen cracked or developed a crack.

The verb треснуть can mean something like:

• to crack
• to split
• to snap

With экран, the natural meaning is that a crack appeared in it. So экран телефона треснул is exactly the kind of thing Russian says for a damaged screen.

Треснул is perfective, so it presents the event as a completed result: the screen ended up cracked.

That fits the sentence well, because it describes what happened after the impact.

If you used an imperfective form, it would suggest an ongoing or repeated process, which is less natural here.

So:

• треснул — cracked, ended up cracked
• imperfective forms would be used in different contexts

This is why треснул is the normal choice in a simple past event like this.

Because Russian has no articles.

English needs words like:

• a
• an
• the

Russian does not. Whether something is definite or indefinite is usually understood from context.

So экран телефона can mean:

• the phone screen
• a phone screen

In this sentence, the context makes it naturally understood as the phone’s screen.

Yes. Russian word order is flexible.

This version:

• После удара экран телефона треснул.

is very natural and puts the circumstance first: after the impact.

You could also say:

• Экран телефона треснул после удара.

That also works, but the focus is a little different. It starts with the main thing that happened: the phone screen cracked.

So both are possible, but the original order is a common way to set the scene first.

Primarily, после удара means after the impact in a time sense.

But in real context, it can also imply cause, because the cracking happened following that impact. So English might understand both:

• after the impact
• as a result of the impact

If you wanted to emphasize cause more directly, Russian could also use expressions with от or other wording. But после удара is perfectly natural here and strongly suggests both sequence and likely cause.

The main stresses are:

• пОсле
• удАра
• экрАн
• телефОна
• трЕснул

So you can say it roughly like this:

ПОсле удАра экрАн телефОна трЕснул.

For a learner, the most useful thing is to remember that Russian stress is mobile and must usually be learned word by word.