Si et fa mal el braç, seu aquí i descansa una estona.

Because Catalan usually expresses physical pain with the pattern fer mal a algú = to hurt someone / to cause pain to someone.

So:

• et = to you
• fa mal = hurts / causes pain
• el braç = the arm

Literally, et fa mal el braç is something like the arm causes pain to you.

This is much more natural in Catalan than using a possessive structure like el teu braç in this context.

Examples:

• Em fa mal el cap = My head hurts
• Li fan mal les cames = His/Her legs hurt
• Ens fa mal l’esquena = Our back hurts

Et is the unstressed object pronoun for you in the singular informal sense (tu).

In this sentence, it means to you.

So:

• em = to me
• et = to you
• li = to him / her / you formal
• ens = to us
• us = to you all
• els = to them

With fer mal, Catalan normally uses this pronoun:

• Em fa mal la mà = My hand hurts
• Et fa mal el braç = Your arm hurts
• Li fa mal l’esquena = His/Her back hurts

Catalan often uses the definite article with body parts when the person is already clear from a pronoun like em, et, li, ens.

So instead of saying your arm, Catalan usually says the arm, because et already tells us whose arm it is.

That is why:

• Et fa mal el braç is natural
• Et fa mal el teu braç is possible in a very emphatic or contrastive context, but not the normal everyday phrasing

This is similar to other Romance languages.

More examples:

• Em rento les mans = I wash my hands
• Li fa mal el coll = His/Her neck hurts
• Ens hem trencat la cama = We broke our leg

Fer mal is a very common Catalan expression meaning:

• to hurt
• to be painful
• to cause pain

The verb is fer (to do / to make), but in this expression it works idiomatically.

So:

• Això fa mal = That hurts
• Et fa mal? = Does it hurt?
• Em fa molt mal = It hurts me a lot / It really hurts

In your sentence, fa is the 3rd person singular present form of fer, because the subject is el braç:

• el braç fa mal = the arm hurts

If the subject were plural, the verb would change:

• Et fan mal els braços = Your arms hurt

Catalan commonly uses the present indicative after si when talking about a real or likely situation in the present or future.

So:

• Si et fa mal el braç, seu aquí = If your arm hurts, sit here

This is the normal structure for practical instructions or advice.

It matches English quite closely:

• If it hurts, rest
• If you are tired, sit down
• If it rains, we’ll stay here

So there is nothing unusual here: si + present is exactly what you would expect.

Seu is the informal singular imperative (tu command) of seure, meaning to sit or to sit down.

So:

• seure = to sit / to sit down
• seu! = sit! / sit down!

In the sentence:

• seu aquí = sit here

Some useful imperative forms of seure:

• (tu) seu
• (vostè) segui
• (vosaltres) seieu
• (vostès) seguin

A learner may notice that this form is not very predictable from the infinitive. That is normal: seure has some irregular forms.

Yes. Seu aquí and asseu-te aquí can both mean sit here / sit down here.

Broadly:

• seure = to sit / sit down
• asseure’s = to sit oneself down / to take a seat

So:

• Seu aquí = Sit here
• Asseu-te aquí = Sit down here / Take a seat here

In everyday use, both are common, though seu aquí is very natural and direct.

A learner may also wonder why there is no pronoun with seu. That is because seure can be used without a reflexive pronoun here, while asseure’s requires one:

• Seu
• Asseu-te

Catalan usually drops subject pronouns unless they are needed for emphasis or contrast.

Also, in commands, the subject is normally omitted.

So Catalan simply says:

• Seu aquí
• Descansa una estona

not usually:

• Tu seu aquí
• Tu descansa una estona

Adding tu would sound emphatic, for example if you were contrasting one person with another.

It means rest for a while or rest a bit.

• descansa = rest
• una estona = a while / a little while / some time

Una estona is a very common expression for an unspecified short period of time.

Examples:

• Espera una estona = Wait a while
• Parlem una estona = Let’s talk for a while
• Descansa una estona = Rest for a while

English often uses for here, but Catalan does not need a preposition in this expression.

I simply means and.

The sentence gives two instructions:

• seu aquí = sit here
• descansa una estona = rest for a while

So:

• seu aquí i descansa una estona = sit here and rest for a while

This is very straightforward coordination, just like in English.

Then both the noun and the verb would become plural:

• Si et fan mal els braços, seu aquí i descansa una estona.

Why?

• els braços = the arms
• fan mal = hurt

Because now the subject is plural.

Compare:

• Et fa mal el braç = Your arm hurts
• Et fan mal els braços = Your arms hurt

This is a useful pattern to remember with fer mal:

• singular thing hurting → fa mal
• plural things hurting → fan mal