Demà la meva germana no correrà perquè encara li fa mal la cama.

Yes. Demà means tomorrow, and putting it at the beginning is very natural in Catalan. It sets the time frame right away:

Demà la meva germana no correrà... = Tomorrow my sister will not run...

Catalan is fairly flexible with word order, especially with time expressions. You could also hear:

La meva germana no correrà demà...

Both are correct, but starting with Demà gives it a little more emphasis.

In Catalan, possessives are usually used together with a definite article. So instead of saying just my sister, Catalan normally says:

la meva germana = literally the my sister

This is standard Catalan grammar:

• el meu germà = my brother
• la meva germana = my sister
• el seu cotxe = his/her car

English does not use the with possessives, but Catalan usually does.

Because germana is a feminine singular noun.

The possessive has to agree with the noun it describes:

• meu = masculine singular
• meva = feminine singular
• meus = masculine plural
• meves = feminine plural

So:

• el meu germà = my brother
• la meva germana = my sister

Correrà is the future tense of córrer = to run.

It means:

correrà = he/she will run

Here it refers to la meva germana, so:

la meva germana no correrà = my sister will not run

A few future forms of córrer are:

• correré = I will run
• correràs = you will run
• correrà = he/she will run
• correrem = we will run
• correreu = you all will run
• correran = they will run

Yes. In Catalan, basic negation is usually made by placing no before the verb:

• corre = she runs
• no corre = she does not run
• correrà = she will run
• no correrà = she will not run

So no works in a very straightforward way, much like not, but without needing an auxiliary like do in the present.

Here perquè means because.

So:

...no correrà perquè encara li fa mal la cama.
= ...will not run because her leg still hurts.

This is different from per què, which usually means why or for what in a question:

• Perquè no vindrà. = Because he/she will not come.
• Per què no vindrà? = Why will he/she not come?

A useful shortcut:

• perquè = because
• per què = why

Here encara means still.

So:

encara li fa mal la cama = her leg still hurts

It shows that the pain has not gone away yet. In other contexts, encara can also mean yet or even even, depending on the sentence, but here still is the right meaning.

Catalan often uses the expression fer mal to say that something hurts.

The pattern is:

A algú li fa mal alguna cosa
= Something hurts someone

So:

• li = to her
• fa mal = hurts / causes pain
• la cama = the leg

Literally, it is something like:

The leg causes pain to her

But the natural English translation is:

Her leg hurts

This structure is extremely common in Catalan:

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

Li is an indirect object pronoun meaning to him, to her, or to you in formal singular.

In this sentence it refers to la meva germana:

encara li fa mal la cama
= her leg still hurts
literally: the leg still causes pain to her

So li tells you who is experiencing the pain.

Compare:

• Em fa mal la cama = My leg hurts
• Et fa mal la cama = Your leg hurts
• Li fa mal la cama = His/Her leg hurts
• Ens fa mal la cama = Our leg hurts
• Us fa mal la cama = Your leg hurts
• Els fa mal la cama = Their leg hurts

Catalan often uses the definite article with body parts when it is already clear whose body part is meant.

So instead of saying her leg, Catalan naturally says:

li fa mal la cama
literally: the leg hurts to her

Because li already tells us that the leg belongs to her, Catalan does not usually need la seva cama here.

This is very common with body parts:

• Em rento les mans = I wash my hands
• Li fa mal el braç = His/Her arm hurts
• Ens vam trencar la cama = We broke our leg

Using la seva cama is possible in some contexts, especially if you need emphasis or contrast, but it is less natural here.

Because the subject is singular: la cama.

The verb fer agrees with what hurts:

• La cama fa mal = The leg hurts
• Les cames fan mal = The legs hurt

So in the sentence:

li fa mal la cama
the singular noun la cama takes singular fa.

If it were plural, you would say:

encara li fan mal les cames
= her legs still hurt

Yes, córrer is the normal Catalan verb for to run.

Its infinitive has an accented vowel: córrer. But in conjugated forms, the stress pattern changes, so the form becomes correrà in the future.

This is normal verb conjugation, not a spelling mistake. Many Catalan verbs change shape when conjugated.

So:

• córrer = to run
• corre = he/she runs
• correrà = he/she will run

Sometimes yes, but in this sentence the future correrà is very natural because it clearly refers to tomorrow.

Catalan can sometimes use the present for future meaning when the context makes it obvious:

• Demà ve = He/She is coming tomorrow

But with no correrà, the simple future sounds very clear and standard:

Demà la meva germana no correrà...

So a learner should definitely be comfortable with this future use.

A too-literal English-based version might lead you toward something less natural, such as trying to say something like la seva cama encara fa mal or avoiding the article with the possessive.

But natural Catalan prefers:

Demà la meva germana no correrà perquè encara li fa mal la cama.

The most important things to notice are:

• Catalan uses article + possessive: la meva germana
• Catalan says fer mal for to hurt
• Catalan uses an indirect object pronoun: li
• Catalan often uses the article with body parts: la cama

Those are the key patterns that make the sentence sound genuinely Catalan.