Si vostè coneix aquesta cançó, podem ballar després de sopar.

Vostè is the formal way to say you in Catalan. It is used for politeness, distance, or respect, similar to formal you in some other languages.

If you were speaking informally to a friend, you would normally use tu instead:

• Si tu coneixes aquesta cançó...

So this sentence sounds polite or somewhat formal because of vostè.

Because vostè takes a third-person singular verb form, not a second-person one.

So:

• tu coneixes = you know, informal
• vostè coneix = you know, formal

This is one of the most important things to remember about vostè: it means you, but grammatically it behaves like he/she.

Yes, often it can.

Catalan usually allows subject pronouns to be left out when the verb already makes the meaning clear. So you could say:

• Si coneix aquesta cançó, podem ballar després de sopar.

That would still be understood as formal you from context, especially in a situation where the speaker is addressing someone politely.

However, keeping vostè makes the formality extra clear.

This is a very common question.

• si without an accent means if
• sí with an accent means yes

So in this sentence, Si vostè coneix... means If you know...

The accent changes the meaning completely.

Catalan normally uses the present indicative after si for a real or possible condition.

So this pattern is normal:

• Si vostè coneix aquesta cançó, podem ballar...

That is like saying:

• If you know this song, we can dance...

You do not normally use a future after si in this kind of sentence. So si vostè coneixerà would be wrong here.

Yes, and the choice changes the tone.

• podem = we can
• podríem = we could

In this sentence, podem sounds more direct and natural if the speaker is simply stating a real possibility:

• If you know this song, we can dance after dinner.

If you said podríem, it would sound a bit more tentative or polite:

• Si vostè coneix aquesta cançó, podríem ballar després de sopar.

So podem is not wrong at all; it just sounds a little more straightforward.

Because cançó is a feminine singular noun, so the demonstrative must agree with it.

• aquest = this, masculine singular
• aquesta = this, feminine singular

So:

• aquest llibre = this book
• aquesta cançó = this song

Agreement is very important in Catalan.

This phrase is very natural in Catalan.

• després de = after
• sopar can mean dinner as a noun, and it is also the infinitive to have dinner / to eat dinner

So després de sopar can be understood as:

• after dinner
• or more literally, after having dinner

Catalan often uses an infinitive after prepositions like després de.

The comma separates the if-clause from the main clause.

• Si vostè coneix aquesta cançó = the condition
• podem ballar després de sopar = the result

This is very similar to English punctuation:

• If you know this song, we can dance after dinner.

You can also reverse the order:

• Podem ballar després de sopar si vostè coneix aquesta cançó.

In that order, a comma is often not needed.

A useful beginner guide is:

• vostè → stress on the last syllable: vos-TÈ
• cançó → stress on the last syllable: can-SÓ
• després → stress on the second syllable: des-PRÉS

A few spelling points:

• The accent marks in vostè, cançó, and després show where the stress goes.
• The ç in cançó sounds like s.

So cançó sounds roughly like kan-SO, not like it has a hard k or z sound at the ç.

Yes. If you want an informal version, you would change both the pronoun and the verb form:

• Si tu coneixes aquesta cançó, podem ballar després de sopar.

Very often, Catalan would also drop tu:

• Si coneixes aquesta cançó, podem ballar després de sopar.

That is probably what you would hear more often in everyday informal speech.