Le verre s'est cassé quand il est tombé sur le carrelage.

In this sentence, le verre most naturally means a drinking glass.

French verre can mean:

• glass as a material
• a glass as an object you drink from

Context tells you which one is meant. Since it fell and broke, the usual interpretation is the drinking glass.

If you wanted to say windowpane, French would usually use vitre, not verre.

Because se casser is a very common way to say that something broke on its own.

• Le verre s'est cassé = The glass broke
• J'ai cassé le verre = I broke the glass

So:

• se casser is used when the subject undergoes the breaking
• casser with a direct object is used when someone breaks something

Using a cassé with le verre as the subject would sound like the glass broke something, which is not the meaning here.

s' is the shortened form of se.

The full verb is se casser.
Before a vowel sound, se becomes s':

• se casse
• s'est cassé

This shortening is called elision. French does it very often:

• je aime → j'aime
• se est → s'est

Both parts are in the passé composé, and both use être as the auxiliary.

1. s'est cassé

   • Pronominal verbs like se casser usually take être in the passé composé.

2. est tombé

   • tomber is one of the common verbs of movement that also takes être in the passé composé.

So the structure is:

• se casser → s'est cassé
• tomber → est tombé

This is why you do not see a cassé or a tombé here.

Because the subject, le verre, is masculine singular.

With verbs that use être in the passé composé, the past participle usually agrees with the subject:

• masculine singular: cassé, tombé
• feminine singular: cassée, tombée
• masculine plural: cassés, tombés
• feminine plural: cassées, tombées

Examples:

• La bouteille s'est cassée.
• Les verres sont tombés.

Here, le verre is masculine singular, so cassé and tombé stay in the basic masculine singular form.

Il refers back to le verre.

French normally uses a subject pronoun once the noun has already been introduced, just as English often does:

• Le verre s'est cassé quand il est tombé...
• The glass broke when it fell...

You could repeat le verre, but it would usually sound heavier and less natural here.

Also, even though verre means glass, it is grammatically masculine, so the pronoun is il, not elle.

Because both actions are seen as completed events in the past:

• the glass broke
• it fell

French often uses the passé composé for a sequence of finished events.

Here, the sentence presents two specific events:

1. it fell on the tile
2. it broke

A learner may expect the imperfect somewhere, but the passé composé is normal because the sentence is talking about concrete completed actions, not background description or habit.

For comparison:

• Le verre s'est cassé quand il est tombé...
  specific event

• Le verre se cassait facilement...
  general tendency / repeated background idea

Here quand means when.

Yes, lorsque would also be possible:

• Le verre s'est cassé lorsque il est tombé...

But in normal French, lorsque before a vowel is usually shortened:

• lorsqu'il

So the smoother version would be:

• Le verre s'est cassé lorsqu'il est tombé sur le carrelage.

Both are correct, but quand is very common and simple in everyday French.

Sur le carrelage means onto/on the tiled floor or on the tiles.

• sur = on / onto
• le carrelage = the tiling / tiled floor / tiles

In this sentence, sur is used because the glass fell and ended up in contact with the tiled surface.

French would not normally say au carrelage here.

Also, carrelage refers to the tiled surface in general, not just one tile.

You may sometimes hear forms like ça a cassé or non-standard uses in speech, but for a learner, the best and most natural pattern is:

• Le verre s'est cassé = The glass broke
• J'ai cassé le verre = I broke the glass

So for this kind of sentence, s'est cassé is the form you should learn and use.

Yes. A common alternative is se briser.

• Le verre s'est cassé = the glass broke
• Le verre s'est brisé = the glass shattered / broke

The difference is mostly nuance:

• se casser is very common and everyday
• se briser can sound a bit more specific or literary, and often suggests a more dramatic break

For a normal spoken sentence, s'est cassé is completely natural.