No se agrietó, sino que se rompió.

Here se is part of the verb forms agrietarse and romperse when they are used intransitively to describe what happened to something.

• agrietar = to crack something

• agrietarse = to crack / to become cracked

• romper = to break something
• romperse = to break / to become broken

So No se agrietó, sino que se rompió does not mean that something cracked itself or broke itself in a literal reflexive sense. It means the object became cracked or became broken.

For example:

• El vaso se rompió. = The glass broke.
• La pared se agrietó. = The wall cracked.

They are similar, but not identical.

• agrietarse means to crack, to develop cracks
• romperse means to break

So the sentence is making a correction:

• It did not merely crack
• It actually broke

That suggests romperse is stronger or more complete here. A thing can be agrietado and still basically remain in one piece, but if it se rompió, it is broken.

This structure is used to correct the first idea and replace it with the true one.

Pattern:

• no X, sino Y = not X, but rather Y
• no X, sino que + clause = not X, but rather + full clause

So:

• No se agrietó, sino que se rompió. = It didn’t crack; rather, it broke.

This is stronger and more precise than pero. It does not just add contrast; it replaces the first statement with the correct one.

Because what follows is a full conjugated clause:

• se rompió

In Spanish:

• sino is used before a word or phrase
• sino que is used before a clause with a finite verb

Compare:

• No es azul, sino verde.
  Not blue, but green.

• No vino Juan, sino que vino Ana.
  It wasn’t Juan who came, but Ana.

In your sentence, se rompió is a full clause, so que is needed.

This is a very common confusion.

• sino = but rather / except
• si no = if not

In your sentence, it must be sino, one word, because the meaning is corrective:

• No se agrietó, sino que se rompió.

If you wrote si no, it would mean something like if not, which does not fit here.

Compare:

• No fue un error, sino una decisión.
  It wasn’t a mistake, but rather a decision.

• Si no estudias, suspenderás.
  If you don’t study, you will fail.

They are in the preterite because the sentence refers to a completed event in the past.

• se agrietó = it cracked
• se rompió = it broke

The speaker is talking about what happened at a particular moment or as a finished result.

If you used the imperfect instead, the meaning would change:

• se agrietaba = it was cracking / it used to crack
• se rompía = it was breaking / it used to break

So the preterite is the natural choice when reporting a single completed event.

Spanish often leaves out the subject pronoun when it is understood from the context or the verb form.

Both agrietó and rompió are third-person singular, so the subject could be:

• él
• ella
• usted
• eso
• some noun understood from context, like el cristal, la mesa, la pantalla, etc.

Spanish does this all the time. English usually needs an explicit subject, but Spanish often does not.

Grammatically, pero is possible in some contrasting contexts, but it would not express exactly the same thing.

• No se agrietó, pero se rompió sounds more like simple contrast:
  It didn’t crack, but it broke.

• No se agrietó, sino que se rompió sounds like a correction:
  It didn’t crack; rather, it broke.

So sino que is better here because the second part replaces the first idea with the correct one.

Yes, they are required.

In the preterite, the third-person singular endings are:

• -ó for -ar verbs
• -ió for -er and -ir verbs

So:

• agrietó
• rompió

The accent mark shows the stress and distinguishes these forms from others.

For example:

• rompio would be incorrect
• rompió is correct

This is standard spelling in Spanish, including Spain.

The comma is very common and natural here, especially because the sentence has two balanced clauses:

• No se agrietó, sino que se rompió.

In short sentences, some writers may omit it, but using the comma is perfectly standard and often clearer.

So for a learner, it is a good idea to keep it.

In natural English, the best match is usually it didn’t crack.

Spanish agrietarse is often best translated with an English intransitive verb:

• La pared se agrietó. = The wall cracked.

You could sometimes translate it as it didn’t get cracked, but that sounds more passive and less natural in many contexts. The Spanish sentence is simply describing what happened to the object, not focusing on who caused it.