Ahir vam seure al balcó amb una manta perquè feia fred.

Vam seure is the very common Catalan periphrastic past:
vam + infinitive

Here:

• vam = we in the past construction
• seure = to sit / to sit down

So vam seure means we sat down or we sat.

This is one of the most common ways to talk about completed past actions in Catalan, especially in everyday language.

Examples:

• Vam menjar = we ate
• Vam arribar = we arrived
• Vam seure = we sat down

Seure means to sit down or sometimes simply to sit, depending on context.

In this sentence, it suggests the action of taking a seat:

• Ahir vam seure al balcó... = Yesterday we sat down on the balcony...

It is related to:

• seure = to sit down / to sit
• estar assegut / asseguda = to be seated / to be sitting

So:

• Vam seure = we sat down
• Estàvem asseguts = we were sitting / we were seated

In standard Catalan, seure is normally used without a reflexive pronoun when it means to sit down.

So the standard form is:

• vam seure

Not:

• ens vam seure in standard usage

This can feel strange to English speakers because many Romance languages use a reflexive form for this idea, but Catalan usually does not here.

Compare:

• Vaig seure a la cadira = I sat down on the chair
• Vam seure al balcó = We sat on the balcony

Because a + el contracts to al in Catalan.

So:

• a + el balcó → al balcó

This is very common.

Other examples:

• Vaig al mercat = I go to the market
• Som al carrer = We are in/on the street

So al balcó means on the balcony or to the balcony, depending on context. Here it means on the balcony.

Prepositions do not always match English exactly.

In Catalan, a is often used where English would use on, in, or at, depending on the place.

So:

• seure al balcó = to sit on the balcony
• ser a casa = to be at home
• anar a l’escola = to go to school

You should think of a as a very flexible location/direction preposition.

Amb una manta literally means with a blanket.

In context, it usually suggests that they were using the blanket to keep warm:

• sitting with a blanket over them
• wrapped in a blanket
• having a blanket with them

It does not necessarily mean they were carrying it in their hands. The context perquè feia fred makes it clear the blanket was for warmth.

Una manta means a blanket, so it introduces the blanket as something unspecified or not previously mentioned.

If you said la manta, that would mean the blanket, referring to a specific blanket that is already known from context.

So:

• amb una manta = with a blanket
• amb la manta = with the blanket

This is a very common question.

• perquè = because / so that
• per què = why / for what

In your sentence:

• perquè feia fred = because it was cold

Compare:

• No vam sortir perquè plovia. = We didn’t go out because it was raining.
• Per què no vau sortir? = Why didn’t you go out?

The accent and spacing matter.

Catalan uses fer in many weather and temperature expressions.

So:

• fa fred = it is cold
• feia fred = it was cold
• fa calor = it is hot
• feia vent = it was windy

This is an impersonal expression. English uses it is cold, but Catalan says something more like it makes cold.

You do not normally say:

• era fred for it was cold in this context

Feia is the imperfect of fer, and it is used because the cold is background information or an ongoing situation in the past.

The sentence has:

• vam seure = completed action
• feia fred = ongoing condition/background

So the structure is very natural:

• Yesterday we sat on the balcony with a blanket because it was cold.

This is a classic contrast:

• completed event → periphrastic past
• background description / ongoing state → imperfect

It does not have to be first. Catalan word order is flexible.

Your sentence begins with Ahir because it sets the time clearly from the start:

• Ahir vam seure al balcó...

But other orders are also possible, depending on emphasis:

• Vam seure al balcó ahir.
• Ahir, al balcó, vam seure amb una manta perquè feia fred.

Beginning with ahir is very natural and common.

Yes. Ahir means yesterday, and the h is silent.

So it is pronounced roughly like:

• a-IR

The stress falls on the second syllable.

Also, unlike Spanish ayer, the Catalan word is completely different, so it is worth memorizing on its own.

No. In this sentence, perquè clearly means because.

Although perquè can sometimes mean so that, context normally makes the meaning obvious. Here:

• vam seure al balcó amb una manta perquè feia fred

The clause feia fred is giving a reason, so the meaning is:

• because it was cold

Not a purpose.

In the periphrastic past, the auxiliary changes according to the subject:

• vaig
  • infinitive = I
• vas
  • infinitive = you (singular)
• va
  • infinitive = he/she/it
• vam
  • infinitive = we
• vau
  • infinitive = you (plural)
• van
  • infinitive = they

So:

• vam seure = we sat

Even if the subject nosaltres is not written, the verb form tells you who the subject is.

Yes, it sounds natural and idiomatic.

Everything in it is very standard:

• Ahir for yesterday
• vam seure for a completed past action
• al balcó with the normal contraction
• amb una manta for with a blanket
• perquè feia fred for because it was cold

It is a good example of natural spoken and written Catalan.