Si el got és buit, la meva filla hi posa més aigua abans de seure a taula.

Here hi means something like in it or there, referring back to el got.

So:

• la meva filla hi posa més aigua = my daughter puts more water in it

Catalan often uses these short clitic pronouns instead of repeating the noun. Without hi, the sentence would lose that explicit idea of into the glass.

In Catalan, possessives are very often used together with the definite article:

• la meva filla = my daughter
• el meu pare = my father
• la nostra casa = our house

So this is the normal pattern. English does not use an article here, but Catalan usually does.

Si means if and introduces a condition:

• Si el got és buit... = If the glass is empty...

It sets up the situation under which the action happens. This is the normal word for real or possible conditions in Catalan.

After si in ordinary real conditions, Catalan uses the indicative, not the subjunctive.

So:

• Si el got és buit, ... = correct
• a subjunctive here would not be the normal choice

This is similar to English If the glass is empty..., not if the glass be empty in normal speech.

This is a very common learner question. In Catalan, ser is often used in cases where an English speaker, or especially a Spanish speaker, might expect estar.

So el got és buit is natural Catalan.
You may also hear el got està buit, especially if the speaker wants to emphasize the current state, but és buit is perfectly normal.

A helpful rule is: do not assume Catalan uses ser/estar exactly like Spanish.

Posa is from posar, which literally means to put, but in everyday Catalan it is used very broadly.

Here hi posa més aigua means she puts/adds more water in it.

Depending on context, English might translate it as:

• puts
• adds
• sometimes even pours

Catalan often prefers this general verb where English uses a more specific one.

Yes. For example:

• hi fica més aigua can also mean she puts more water in it
• hi aboca més aigua is closer to she pours more water into it

But posar is very common and neutral, so it is a very natural choice here.

Because aigua is being used as an indefinite mass noun, like water in English.

• més aigua = more water

You are not talking about a specific, already identified portion of water, just more water in general.

That is why Catalan does not use an article here.

Because hi is a clitic pronoun, and in Catalan clitics usually go before a conjugated verb:

• hi posa
• la hi posa
• no hi posa

If the verb were an infinitive, the pronoun would usually attach after it:

• posar-hi

So the position of hi is part of normal Catalan clitic placement.

Sometimes in conversation, if the context is very clear, a speaker might leave it out. But in this sentence hi is useful and natural because it clearly tells you where the water is being put: into the glass.

Without hi, posa més aigua would just mean puts/adds more water, with the destination left unstated.

After abans de, Catalan uses an infinitive when the subject is the same as in the main clause.

Here the subject is la meva filla throughout:

• la meva filla hi posa més aigua
• before seure a taula

So abans de seure means before sitting down.

This works much like English before sitting down.

The understood subject is the same as the subject of the main clause: la meva filla.

So the meaning is:

• my daughter puts more water in it before she sits down at the table

Catalan often leaves that subject unspoken when the infinitive structure already makes it clear.

A taula is a very common idiomatic expression meaning at the table, especially in the sense of sitting down for a meal.

So:

• seure a taula = to sit down at the table / to sit down to eat

By contrast, seure a la taula sounds more purely physical or literal: sitting at a particular table.

In many contexts both are possible, but a taula is especially natural when talking about meals.

Yes, the present tense here most naturally suggests a habitual or repeated action:

• If the glass is empty, my daughter puts more water in it before sitting down at the table.

It sounds like something she generally does, not just one unique event.

Catalan, like English, often uses the present tense for routines and repeated behavior.

Because Si el got és buit is an introductory conditional clause, and the main clause follows after it:

• Si el got és buit, / la meva filla hi posa més aigua...

This punctuation is very similar to English. When the if-clause comes first, a comma is normally used.

Because adjectives in Catalan agree with the noun they describe.

• got is masculine singular
• so the adjective is buit

Compare:

• el got és buit = masculine singular
• la tassa és buida = feminine singular
• els gots són buits = masculine plural
• les tasses són buides = feminine plural

So buit matches got.

Both forms can be encountered, depending on variety and style, but seure is a standard and very common verb meaning to sit down / to be seated.

So:

• abans de seure a taula = before sitting down at the table

You may also hear forms related to asseure or asseure's, but seure here is completely natural and idiomatic.