Aquesta cassola és massa petita; quan és plena, no hi cap més aigua.

Because cassola is a feminine singular noun in Catalan, so the demonstrative has to agree with it.

• aquest = this (masculine singular)
• aquesta = this (feminine singular)

So:

• aquest llibre = this book
• aquesta cassola = this pot / this casserole dish

Catalan adjectives and determiners usually agree in gender and number with the noun.

For the same reason: they must agree with cassola, which is feminine singular.

• petit = small (masculine)
• petita = small (feminine)
• ple = full (masculine)
• plena = full (feminine)

So:

• un got petit = a small glass
• una cassola petita = a small pot

And:

• el got és ple = the glass is full
• la cassola és plena = the pot is full

Here massa means too or too much / excessively.

So:

• és massa petita = it is too small

This is different from molt:

• molt petita = very small
• massa petita = too small

That distinction is important:

• molt just describes degree
• massa implies more than is acceptable or useful

Because the sentence has two separate clauses:

1. Aquesta cassola és massa petita
2. quan és plena, no hi cap més aigua

In the second clause, és plena means it is full. Catalan often omits subject pronouns when they are understood from the verb and context.

So quan és plena literally means:

• when [it] is full

The subject is still the cassola, but Catalan does not need to repeat ella.

Both ser and estar can appear with adjectives in Catalan, but with ple/plena Catalan commonly uses ser in many ordinary contexts.

So quan és plena is perfectly natural and idiomatic.

For an English speaker, this can feel unusual because English uses to be only once, while Catalan divides some uses between ser and estar differently from Spanish and differently from English expectations.

The key point for this sentence is simple:

• és plena = is full

Very literally, it means something like:

• no = not
• hi = in it / there
• cap = fits
• més aigua = more water

So the whole phrase means:

• no more water fits in it
• there is no room for more water in it

This is a very natural Catalan way to express the idea of physical capacity.

No. Here cap is the verb form of cabre, which means to fit or to have room for.

This is a very common point of confusion, because cap can also be a different word in other contexts, such as:

• cap persona = no person / nobody
• cap = head

But in no hi cap més aigua, cap is definitely the verb:

• cabre = to fit
• cap = fits

So:

• No hi cap més aigua = No more water fits in it

Hi is a locative pronoun. Here it means something like there or in it.

In this sentence, hi refers back to the cassola.

So:

• No hi cap més aigua = No more water fits in it = No more water fits there

Catalan often uses hi where English would use a prepositional phrase such as:

• in it
• there
• inside it

This little word is extremely common in Catalan.

Because cabre usually needs a sense of where something fits.

In this sentence, the idea is fit in the pot. Catalan expresses that with hi:

• hi cap = fits there / fits in it

Without hi, the sentence would sound incomplete or unnatural in this context, because the location is an essential part of the meaning.

Because més aigua simply means more water.

You use més + noun directly:

• més pa = more bread
• més vi = more wine
• més aigua = more water

You do not need de here.

In the sentence:

• no hi cap més aigua = no more water fits in it

Because Catalan often uses mass nouns like aigua without an article when speaking in a general or indefinite sense.

Here it means:

• more water not
• the more water

Compare:

• Vull aigua = I want water
• No hi cap més aigua = No more water fits

If you added an article, the meaning would change or sound unnatural in this sentence.

The verb is cabre, meaning to fit.

Yes, it is somewhat irregular, and learners usually meet it first in forms like:

• hi cap = it fits there / in it
• no hi cap = it doesn’t fit there / in it

In your sentence, cap is the 3rd person singular present form, agreeing with més aigua as the thing that would fit.

So the grammar is:

• més aigua = subject
• cap = fits

That would mean something different.

• no hi cap més aigua = no more water fits in it
• no hi ha més aigua = there is no more water there / in it

So:

• cabre talks about space/capacity
• haver-hi talks about existence/presence

In this sentence, the point is not that the water is unavailable, but that the pot has no room left. That is why cabre is the right verb.

The semicolon joins two closely related ideas:

1. This pot is too small
2. When it is full, no more water fits in it

It works a bit like a strong comma or a softer full stop. It is mainly a punctuation choice, not a special grammar structure.

You could also see similar sentences written with a comma or as two separate sentences, but the semicolon neatly links the cause and result.

The first part, quan és plena, is a time clause:

• quan = when
• és plena = it is full

Then comes the main clause:

• no hi cap més aigua = no more water fits in it

So the structure is:

• When it is full, no more water fits in it

This order is very natural in Catalan. You could also imagine the clauses in a different order in another context, but this one is straightforward and common.