Quan cuino, sempre em poso el davantal i estenc les estovalles sobre la taula.

Because after quan when it means when in a real or habitual situation, Catalan normally uses a conjugated verb, not the infinitive.

• quan cuino = when I cook
• cuinar = to cook

So quan cuino is the natural way to say that this happens whenever you cook.

Cuino is the 1st person singular present tense of cuinar.

• jo cuino = I cook / I am cooking

In this sentence, the present tense is being used for a habitual action:

• Quan cuino, sempre... = When I cook, I always...

The word sempre helps show that this is something you do regularly, not just right now.

Because posar-se is often used for putting something on yourself, especially clothes or wearable items.

• posar = to put, place
• posar-se = to put on oneself

So:

• em poso el davantal = I put on the apron

The pronoun em means myself here and matches jo:

• em = for I
• et = for you
• es = for he/she/it
• ens = for we
• us = for you all
• es = for they

Without em, poso would usually sound more like I place something somewhere.

In normal Catalan affirmative sentences, object and reflexive pronouns usually go before the conjugated verb.

So:

• em poso
• et lleves
• es vesteix

This is different from English, where the equivalent idea often comes after the verb, as in I put on.

With infinitives, gerunds, and affirmative commands, the position can change, but in a simple present-tense sentence like this, em goes before poso.

Catalan often uses the definite article with clothing, body parts, and familiar objects in places where English might use a/an, my, or sometimes no article at all.

So:

• em poso el davantal literally looks like I put on the apron
• but it naturally means I put on my/the apron

This is very common in Catalan and other Romance languages. It does not necessarily mean a specific apron previously mentioned; it can simply be the normal apron used in that situation.

Yes, this is a very common point of confusion.

In Catalan, estovalles is normally used in the plural, even when English would often say tablecloth in the singular.

So:

• les estovalles = the tablecloth / the table linen

You should learn it as a word that is very often plural in Catalan. Native English speakers often expect a singular form here, but les estovalles is the usual expression.

Because estendre is an irregular verb.

Its infinitive is:

• estendre = to spread out, lay out

But in the 1st person singular present, it becomes:

• jo estenc

Some other forms are:

• tu estens
• ell/ella estén
• nosaltres estenem
• vosaltres esteneu
• ells/elles estenen

So estenc les estovalles means I spread out / lay out the tablecloth.

Sobre here means on or over, and in this sentence it means on the table.

• sobre la taula = on the table

Catalan can use sobre in this kind of physical sense. English learners may expect a one-to-one match with English prepositions, but prepositions do not always line up perfectly between languages.

In this sentence, sobre la taula is completely natural for saying that the tablecloth is spread out onto the surface of the table.

Yes, adverbs like sempre can sometimes move, but its position here is very natural.

• Quan cuino, sempre em poso el davantal...

This placement clearly modifies the whole action: whenever I cook, I always do these things.

Catalan word order is somewhat flexible, but this version sounds very standard and natural. If you move sempre, the sentence may still be grammatical, but this placement is one of the most common choices.

Because Quan cuino is an introductory subordinate clause:

• Quan cuino, ... = When I cook, ...

The comma helps separate the introductory time clause from the main clause. In practice, punctuation can vary a little, but using the comma here is very normal and clear.

Yes, but it would not mean exactly the same thing in tone.

• Quan cuino usually means when I cook in a general or habitual sense.
• Quan estic cuinant means more specifically when I am cooking or while I’m cooking, focusing more on the action in progress.

In your sentence, Quan cuino is the better choice because the sentence describes a routine:

• When I cook, I always put on an apron and spread out the tablecloth.

Not exactly.

• estendre les estovalles means to spread out / lay out the tablecloth
• parar la taula means to set the table

So this sentence specifically mentions putting the tablecloth on the table, not the whole action of setting places, plates, cutlery, and so on.

In some contexts people may understand you, but it is not the best choice here.

• em poso el davantal = I put on the apron
• poso el davantal more literally suggests I place the apron

Since the apron is something you are wearing, Catalan normally prefers the reflexive form posar-se. So sempre em poso el davantal is the idiomatic version.