Demà treballo a l'escola, però avui no treballo.

In Catalan, just like in English, the present tense can be used for a future meaning when a time expression makes it clear.

So:

• Demà treballo = Tomorrow I’m working / Tomorrow I work

Because demà already tells you the action is in the future, Catalan does not need a special future form here.

You could also use the future tense:

• Demà treballaré

But demà treballo sounds very natural for planned or scheduled actions.

Catalan often drops the subject pronoun when it is clear from the verb ending.

• treballo means I work
• The -o ending tells you the subject is jo = I

So:

• (Jo) treballo = I work

The pronoun jo can be added for emphasis, contrast, or clarity, but it is usually unnecessary.

The infinitive is treballar, which means to work.

Here is the relevant form:

• treballar = to work
• treballo = I work

So the sentence uses the first person singular present tense of treballar.

a l'escola means at the school or to the school, depending on context. In this sentence, it means at the school.

It is written as:

• a = at / to
• l' = the
• escola = school

So a + l'escola gives a l'escola.

The article becomes l' before a vowel sound. This is similar to elision in other Romance languages.

Compare:

• a l'escola = at the school
• a la feina = at work
• al despatx = at the office

Catalan definite articles change form depending on gender, number, and whether the next word begins with a vowel.

For a feminine singular noun, the normal article is la:

• la casa = the house

But before many vowel sounds, la becomes l':

• l'escola = the school
• l'amiga = the friend

So escola takes l' instead of la because it begins with a vowel.

It can mean both, depending on context.

Catalan simple present often covers what English expresses with either:

• I work
• I am working

So:

• avui no treballo can mean I’m not working today or I don’t work today, though in this context I’m not working today is the most natural English translation.

The time words demà and avui help determine the meaning.

Yes, you absolutely can say:

• Demà treballo a l'escola, però avui no.

That is natural and means Tomorrow I work at the school, but not today.

In the original sentence, treballo is repeated because it makes the contrast very clear and complete:

• Demà treballo..., però avui no treballo.

Both versions are correct. Repeating the verb is slightly fuller and more explicit.

però means but.

It connects two contrasting ideas:

• Demà treballo a l'escola = Tomorrow I work at the school
• avui no treballo = today I’m not working

So però shows the contrast between tomorrow and today.

The accent in però is a written stress mark. It helps show the correct pronunciation and distinguish the word according to Catalan spelling rules.

• però = but

In speech, the stress falls on the last syllable: pe-RÒ.

Catalan uses accent marks more regularly than English, so learners will see them often.

A simple learner-friendly pronunciation guide is:

• Demà ≈ de-MAH
• treballo ≈ truh-BA-lyu / truh-BA-yo (depending on accent)
• a l'escola ≈ ah ləs-KO-lə
• però ≈ pe-RO
• avui ≈ ah-VUI
• no treballo ≈ no truh-BA-lyu

A few useful notes:

• ll in many Catalan varieties sounds like a palatal sound, somewhat like ly.
• v and b are often pronounced very similarly.
• Final stressed vowels are important, especially in demà and però.

Pronunciation varies somewhat across Catalan-speaking regions, but this will guide you well.

Because però introduces a contrasting clause, Catalan normally uses a comma before it, just as English usually does before but in a sentence like this.

So:

• Demà treballo a l'escola, però avui no treballo.

The comma helps separate the two contrasted parts of the sentence.

Yes, a can mean to or at, depending on context. But in this sentence, at the school is the natural meaning because it goes with treballo.

• treballo a l'escola = I work at the school

If the verb involved movement, a l'escola could mean to the school:

• Vaig a l'escola = I go to school / I go to the school

So the verb helps determine whether a means to or at.

Yes, you can, but it is usually not necessary.

• Demà treballo a l'escola = normal, neutral
• Demà jo treballo a l'escola = possible, but more emphatic

Adding jo can sound like:

• emphasis
• contrast
• clarification

For example:

• Ell no treballa demà, però jo treballo.
  He isn’t working tomorrow, but I am.

So in the original sentence, leaving out jo is the most natural choice.