Abans de dinar, estenc les estovalles sobre la taula.

Because abans de is the normal Catalan pattern for before.

After de, Catalan often uses an infinitive:

• abans de dinar = before having lunch / before lunch

If you want to treat lunch more clearly as a noun, you can also say:

• abans del dinar = before the lunch meal

So in this sentence, dinar is being used in a verbal way.

It can mean both, depending on context.

• As a verb, dinar = to have lunch
• As a noun, dinar = lunch

Here, after abans de, learners usually understand it as the infinitive:

• abans de dinar = before having lunch

But in natural English, that is often just translated as before lunch.

Estenc is the 1st person singular present form of the verb estendre.

So:

• estendre = to spread / lay out
• estenc = I spread / I lay out

This verb is a bit irregular, so the jo form is not built in the most predictable way. That is something you simply have to learn as part of the conjugation.

Because Catalan usually drops subject pronouns when the verb ending already shows who the subject is.

• estenc already means I spread
• so jo estenc is usually unnecessary

You can add jo for emphasis or contrast:

• Jo estenc les estovalles, no tu.

But in a neutral sentence, leaving it out is the most natural choice.

Because estovalles is very commonly used as a plural noun in Catalan, even when English would often say the tablecloth in the singular.

So:

• les estovalles often corresponds to English the tablecloth

This is one of those cases where you should not translate word for word. The Catalan noun is commonly plural in everyday usage.

Catalan uses definite articles very naturally with specific everyday objects.

So:

• les estovalles = the tablecloth
• la taula = the table

In this sentence, the speaker is referring to the tablecloth and the table involved in the situation, so the definite articles are the normal choice.

Because sobre clearly means on / on top of / onto the surface of.

That fits the action of spreading something on a table:

• estenc les estovalles sobre la taula = I spread the tablecloth on the table

By contrast, a la taula often means:

• at the table
• to the table

So sobre la taula is much more precise here.

It is very natural and standard to put a comma after an introductory time expression like Abans de dinar.

So:

• Abans de dinar, estenc les estovalles sobre la taula.

This helps mark the pause and makes the sentence easier to read. In some short sentences, people may omit the comma, but with it the sentence looks polished and clear.

Yes. Catalan allows some flexibility in word order.

For example, you could also say:

• Estenc les estovalles sobre la taula abans de dinar.

That is still grammatical. The original version puts the time expression first:

• Abans de dinar, ...

This is very natural because it sets the scene first: before lunch.

Not exactly.

Estendre here means:

• to spread
• to lay out

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

That is more specific than English set the table, which can include plates, cutlery, glasses, and so on. This Catalan sentence focuses on putting the tablecloth on the table.