Si estàs lliure demà, podem quedar després de la classe.

Because Catalan uses estar for temporary states or situations, and ser for more permanent identity or classification.

Here, estar lliure means to be free / available at a particular time, so Si estàs lliure demà... means If you’re free tomorrow...

Compare:

• Estàs lliure demà? = Are you free tomorrow?
• Ets lliure = You are free, in a broader or more abstract sense

So in this sentence, estàs is the natural choice.

Here lliure means free, in the sense of available or not busy.

So estàs lliure demà means something like:

• Are you free tomorrow?
• Are you available tomorrow?

It does not mean free as in costing no money. For that, Catalan would usually use gratis or another expression depending on context.

Also, lliure is an adjective:

• singular: lliure
• plural: lliures

In the singular, masculine and feminine look the same.

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

• estàs = you are
• podem = we can

So Catalan does not need to say tu estàs or nosaltres podem unless there is special emphasis, contrast, or clarity needed.

This is very normal in Catalan:

• Si estàs lliure... = If you’re free...
• Podem quedar... = We can meet...

If you added the pronouns, it would sound more emphatic:

• Si tu estàs lliure demà, nosaltres podem quedar...

That is possible, but less neutral.

Si means if.

In Catalan, after si in a real or possible condition, you normally use the indicative, not the future.

So this is correct:

• Si estàs lliure demà, podem quedar...

Not:

• Si estaràs lliure demà...

This is similar to English, where we say:

• If you are free tomorrow, we can meet
• not usually If you will be free tomorrow...

So the structure is:

• si + present indicative
• then a main clause with whatever tense fits the meaning

Here the main clause is podem quedar, which expresses a possible plan.

Literally, podem means we can.

In context, though, it often works like a suggestion, so in English it may feel closer to either:

• we can meet
• we could meet

So podem quedar is a natural, fairly neutral suggestion.

If you wanted to sound a bit more tentative or softer, Catalan could also use:

• podríem quedar = we could meet

So:

• podem quedar = straightforward suggestion
• podríem quedar = slightly more tentative/polite

Here quedar means to meet up or to arrange to meet.

That is a very common use of quedar in Catalan.

So:

• podem quedar després de la classe = we can meet up after class

Be careful: quedar can mean different things in other contexts, such as to remain, to be left, or to fit/look. But in this sentence, it clearly means meet.

You will also often see:

• quedar amb algú = to meet someone

For example:

• He quedat amb la Marta = I’m meeting Marta / I arranged to meet Marta

Because when després is followed by a noun phrase or an infinitive, it normally takes de.

So:

• després de la classe = after class / after the class
• després de dinar = after eating / after lunch

But després on its own means afterwards or later:

• Ara no; després. = Not now; later.

So in this sentence, de is needed because després is followed by la classe.

Yes, both can exist, but there is a nuance.

• després de la classe usually points to a specific class
• després de classe can sound more general, like after class as part of a routine

In your sentence, després de la classe suggests a particular class that both speakers know about.

So the sentence feels like:

• If you’re free tomorrow, we can meet after the class

Whereas després de classe can feel a bit more general:

• after class
• once class is over

Both are understandable, but després de la classe is more specific.

Yes. Demà is fairly flexible.

You could say:

• Si estàs lliure demà, podem quedar després de la classe.
• Si demà estàs lliure, podem quedar després de la classe.

Both are natural. The difference is mainly one of emphasis and rhythm.

The original sentence is very normal and smooth. Putting demà right after lliure keeps the idea together: free tomorrow.

They mainly show where the stress falls, and in some cases they also show vowel quality.

In this sentence:

• estàs → stress on the last syllable
• demà → stress on the last syllable
• després → stress on the last syllable

For English speakers, the most useful thing is to remember that the accents help you pronounce the word with the correct stress.

So roughly:

• es-TÀS
• de-MÀ
• des-PRÉS

Without the accents, a learner might stress the wrong syllable.

Because in Catalan, de + la stays de la.

Some combinations do contract:

• de + el = del
• a + el = al
• de + els = dels
• a + els = als

But with la, there is no contraction:

• després de la classe
• not després dela classe

So de la is exactly what you should expect here.

It is very natural and standard to put a comma after the if-clause when it comes first:

• Si estàs lliure demà, podem quedar després de la classe.

The comma helps separate the condition from the main clause.

In very informal writing, people sometimes leave it out, especially in short sentences, but using the comma is the safer and more standard choice.

So yes: the comma here is a good, normal choice.