Ahir no vaig cuinar a casa perquè ja vam sopar al restaurant.

These are forms of the Catalan periphrastic past, a very common way to talk about completed actions in the past.

• vaig cuinar = I cooked
• vam sopar = we had dinner / we ate dinner

The pattern is:

• vaig / vas / va / vam / vau / van
  • infinitive

So:

• vaig cuinar = I cooked
• vam sopar = we had dinner

For many learners, this looks strange because vaig normally means I go when it stands alone. But in this structure, it is just part of the past tense.

Here, vaig and vam are auxiliary forms used to build the past tense.

• vaig = first person singular auxiliary for this past tense
• vam = first person plural auxiliary for this past tense

So:

• no vaig cuinar = I didn’t cook
• ja vam sopar = we already had dinner

Even though they come from the verb anar (to go), in this sentence they do not mean go.

In Catalan, no normally goes before the conjugated verb.

So the normal structure is:

• no vaig cuinar = I didn’t cook

Not:

• vaig no cuinar

This is similar to how Catalan places negation before the finite verb. Since vaig is the conjugated part, no comes before it.

Ahir means yesterday, and it sets the time for the whole sentence.

Putting it at the beginning is very natural because it gives the listener the time frame right away:

• Ahir no vaig cuinar... = Yesterday I didn’t cook...

Catalan often places time expressions at the beginning, but other positions are also possible, for example:

• No vaig cuinar ahir a casa...

That said, the version with Ahir first sounds very natural and common.

Catalan often drops subject pronouns when the verb form already makes the subject clear.

So:

• (jo) no vaig cuinar
• (nosaltres) vam sopar

The pronouns are usually omitted unless you want to add emphasis, contrast, or clarity.

For example:

• Jo no vaig cuinar; ell sí.
  = I didn’t cook; he did.

In your sentence, the verb forms already clearly show I and we, so pronouns are unnecessary.

Here, perquè means because.

So:

• ...perquè ja vam sopar al restaurant.
• ...because we had already had dinner at the restaurant.

A useful distinction is:

• perquè = because
• per què = why / for what reason

So in this sentence, it is the conjunction because.

Ja often means already.

So:

• ja vam sopar = we already had dinner

It adds the idea that the dinner had already happened, which explains why the speaker did not cook at home.

Depending on context, ja can sometimes have slightly different nuances, but already is the most natural translation here.

This is a very common thing learners notice.

a casa

In Catalan, casa often works almost like home in English.

• a casa = at home

You usually do not use an article here.

al restaurant

This is:

• a + el restaurant → al restaurant

So al is a contraction meaning at the restaurant or to the restaurant, depending on context.

So:

• a casa = at home
• al restaurant = at the restaurant

Sopar can be both a noun and a verb.

As a verb, it means:

• to have dinner
• sometimes to eat dinner

So:

• vam sopar = we had dinner

As a noun:

• el sopar = the dinner / the evening meal

In your sentence, sopar is clearly the verb, because it comes after vam, the past-tense auxiliary.

Because that is how the periphrastic past is formed in Catalan:

• conjugated auxiliary
  • infinitive

So:

• vaig cuinar
• vam sopar

Not:

• vaig cuinat
• vam sopat

This is one of the key patterns to learn. In this tense, the main verb stays in the infinitive.

Not perfectly. Some parts match directly, but some need more natural English wording.

Word by word, you get something like:

• Ahir = yesterday
• no vaig cuinar = I didn’t cook
• a casa = at home
• perquè = because
• ja vam sopar = we already had dinner
• al restaurant = at the restaurant

A natural English translation would be:

• Yesterday I didn’t cook at home because we had already had dinner at the restaurant.

Or more simply:

• Yesterday I didn’t cook at home because we had already eaten at the restaurant.

So the sentence is quite close to English, but you should think in Catalan structures, not just translate word for word.

Yes. In standard Catalan, vam and vàrem are two versions of the same first-person plural past auxiliary.

So both of these mean the same thing:

• vam sopar
• vàrem sopar

In most everyday modern Catalan, especially in many standard contexts, vam is very common.

Yes, but vaig cuinar and vam sopar are extremely natural and standard here.

Catalan also has compound forms like:

• he cuinat
• hem sopat

But in much of standard Catalan, especially when talking about a finished event in the past such as yesterday, the periphrastic past is the most natural choice:

• Ahir no vaig cuinar...

So for this sentence, the tense used is exactly what learners should expect to see often.