Si es trenca una esponja, n'hem de comprar una altra abans d'esbandir els gots.

Because trencar is usually transitive: it means to break something.

• Trenco el got = I break the glass

But here the meaning is a sponge breaks / gets broken, so Catalan commonly uses the pronominal form:

• La esponja es trenca = The sponge breaks

So es trenca una esponja means if a sponge breaks.

In other words:

• trencar = to break something
• trencar-se = to break, to get broken

The es here is not really himself / herself / itself in the English sense. It is part of the verb form.

Catalan often allows the subject to come after the verb, especially when the subject is indefinite or introduces something new.

So both of these are possible:

• Si una esponja es trenca
• Si es trenca una esponja

The version in your sentence sounds very natural. It is a bit like saying:

• If a sponge breaks
• literally, something like If breaks a sponge

English usually requires the subject before the verb, but Catalan is more flexible.

N' is the weak pronoun en, which often means of it / of them / some / one depending on context.

Here it replaces una esponja or, more precisely, the idea of another sponge. So:

• Hem de comprar una altra esponja = We have to buy another sponge
• N'hem de comprar una altra = We have to buy another one

So n' here is basically the Catalan way of saying one in buy another one, though grammatically it works differently from English.

Both patterns can exist with en, but in this kind of sentence Catalan very often places the weak pronoun before the conjugated verb:

• N'hem de comprar una altra

This is the most standard placement here.

You may also see pronouns attached to an infinitive in other structures, but with haver de + infinitive, putting the pronoun before the conjugated verb is very common and very natural.

So for a learner, n'hem de comprar una altra is an excellent model to follow.

Hem de + infinitive means we have to or we must.

• hem = we have
• de comprar = to buy

So:

• hem de comprar = we have to buy

This is a very common Catalan structure for obligation:

• He de marxar = I have to leave
• Has d'estudiar = You have to study
• Han de venir = They have to come

Una altra means another one or another.

Here:

• una = one / a
• altra = other / another

Because esponja is feminine, the form is feminine too:

• una altra esponja = another sponge

In your sentence, the noun esponja is omitted because it is already understood:

• n'hem de comprar una altra = we have to buy another one

If the noun were masculine, you would get:

• un altre got = another glass

Normally, no. In this sentence, una altra is the natural form because it means another one.

Compare:

• una altra esponja = another sponge
• una altra = another one

Leaving out una would sound incomplete here.

Because abans normally takes de before an infinitive.

• abans de sortir = before leaving
• abans de menjar = before eating
• abans d'esbandir = before rinsing

The de becomes d' before a vowel:

• de + esbandir → d'esbandir

So this is just a normal contraction.

Because after abans de, Catalan uses the infinitive when the subject is general or understood.

So:

• abans d'esbandir els gots = before rinsing the glasses

This works like English before + -ing or before + infinitive idea, but Catalan uses the infinitive.

More examples:

• abans de dormir = before sleeping
• abans de començar = before starting

Esbandir means to rinse.

It is more specific than rentar:

• rentar = to wash
• esbandir = to rinse

So if you wash glasses with soap and water, esbandir is the step where you rinse the soap off.

Catalan often uses the definite article where English might use the, some, or even no article depending on context.

Here els gots means the glasses, probably the glasses that are already understood in the situation.

That is very normal in Catalan. The article often refers to things that are contextually known, not only things already explicitly mentioned.

Because si meaning if is written without an accent.

Catalan distinguishes:

• si = if
• sí = yes

So in your sentence:

• Si es trenca una esponja... = If a sponge breaks...

Yes. This is a normal real condition:

• Si es trenca una esponja, n'hem de comprar una altra.

That means something like:

• If a sponge breaks, we have to buy another one.

Catalan often uses the present tense in both parts for general truths, habits, or likely situations, just as English can.

Catalan frequently uses apostrophes when certain short words come before a vowel sound.

Here:

• en + hem → n'hem
• de + esbandir → d'esbandir

This is a normal spelling rule to avoid awkward vowel sequences.

So the apostrophes are not optional; they are part of correct written Catalan.

Yes, the word is very similar and means the same thing: a sponge.

The main thing to notice here is the pronunciation and the surrounding Catalan grammar, not the basic meaning of the noun.

Yes, that is also grammatical and natural.

It keeps the same meaning:

• Si una esponja es trenca, hem de comprar-ne una altra abans d'esbandir els gots.

Differences:

• Si una esponja es trenca puts the subject first.
• hem de comprar-ne places en with the infinitive instead of before the conjugated verb.

The original sentence is still very idiomatic, but your alternative is also good Catalan.