Si la tos és pitjor a la nit, has de tornar a l'hospital.

In Catalan, si without an accent means if.

• si = if
• sí = yes

So in this sentence, Si la tos és pitjor a la nit... means If the cough is worse at night...

The accent matters because it changes the meaning.

És is the verb to be in the he/she/it is form.

• és = is
• es = a pronoun, often like oneself / himself / herself / itself or part of a pronominal verb

So:

• la tos és pitjor = the cough is worse

The accent helps distinguish the verb from the pronoun.

Yes. Tos (cough) is a feminine noun in Catalan, so it takes la.

• la tos = the cough

This may feel unusual to an English speaker because English nouns do not have grammatical gender. In Catalan, you simply have to learn the noun together with its article.

Pitjor means worse.

It is a comparative form, like English better / worse rather than more good / more bad.

So:

• la tos és pitjor = the cough is worse

You can think of pitjor as the normal word Catalan uses for worse.

Because pitjor is the usual comparative word for worse.

Just as English normally says worse, not more bad, Catalan normally says pitjor, not més dolenta.

With symptoms or conditions, pitjor is very natural:

• La febre és pitjor = The fever is worse
• El dolor és pitjor = The pain is worse

A la nit means at night.

Catalan often uses a in time expressions where English uses at.

So:

• al matí = in the morning
• a la tarda = in the afternoon
• a la nit = at night

This is just the normal Catalan way to express that time period.

Catalan often leaves out subject pronouns because the verb ending already shows who the subject is.

• has = you have

So has de tornar already means you have to return / you must go back.

You could say tu has de tornar, but tu is usually omitted unless you want emphasis or contrast.

No. Haver de + infinitive is a standard Catalan structure meaning to have to / must.

So:

• has de tornar = you have to return / you must go back

Even though de often means of or from in other contexts, here it is simply part of the obligation structure:

• he de = I have to
• has de = you have to
• ha de = he/she/it has to

This is very common and important in Catalan.

In standard Catalan, haver de + infinitive is the preferred form for obligation.

• has de tornar = standard, natural Catalan

Some speakers may use forms influenced by Spanish, but if you are learning standard Catalan, haver de is the one to learn and use.

Tornar a can mean either:

1. to return / go back to a place, or
2. to do something again, depending on what follows

In this sentence, because it is followed by a place, it means to go back to / return to:

• tornar a l'hospital = return to the hospital

Compare:

• tornar a casa = return home
• tornar a trucar = call again

So the context tells you which meaning is intended.

Because el becomes l' before a vowel sound.

• el hospital → l'hospital

This is called elision, and it is very common in Catalan:

• l'escola = the school
• l'amic = the friend
• l'hospital = the hospital

The h in Catalan is silent, so hospital begins with a vowel sound for this purpose.

That a is the preposition to.

• tornar a l'hospital = return to the hospital

So the sentence contains two different a's with different jobs:

• a la nit = at night
• a l'hospital = to the hospital

This is normal in Catalan.

Catalan often uses the present tense in real, likely conditions, just as English often does:

• Si la tos és pitjor... = If the cough is worse...
• has de tornar... = you have to go back...

This is a normal if + present, present/obligation pattern.

It expresses a real condition and the action that should follow.

Yes, it is normal to put a comma after the initial conditional clause:

• Si la tos és pitjor a la nit, has de tornar a l'hospital.

This helps separate:

1. the condition: Si la tos és pitjor a la nit
2. the result/instruction: has de tornar a l'hospital

It works much like English punctuation in similar sentences.

A fairly literal breakdown is:

• Si = if
• la tos = the cough
• és = is
• pitjor = worse
• a la nit = at night
• has de = you have to
• tornar = return / go back
• a l'hospital = to the hospital

So very literally:

If the cough is worse at night, you have to return to the hospital.

Yes. A la nit indicates the time when the symptom becomes worse.

So la tos és pitjor a la nit means the cough is worse during the night / at night.

Depending on context, English might translate it as:

• worse at night
• worse during the night

Both capture the idea well.