Quan sortim per la porta, la meva mare apaga la llum.

In Catalan, the verb ending usually already tells you who the subject is. Sortim means we leave / we go out, so nosaltres is not necessary.

• surto = I leave
• surts = you leave
• sortim = we leave

You can add nosaltres for emphasis or contrast, but normally it is left out: Quan sortim per la porta... = When we leave through the door...

Sortim is the 1st person plural present tense of sortir, meaning to leave, to go out, or to exit.

So:

• sortir = to leave / to go out
• sortim = we leave / we go out

In this sentence, it refers to we in a general or habitual sense.

Catalan often uses the present tense in time clauses with quan just like English often does:

• When we leave, my mother turns off the light.

This can describe:

1. a habitual action: every time we leave, she turns off the light
2. a future situation understood from context: when we leave later, she’ll turn off the light

So Quan sortim... is completely natural.

Quan means when and introduces a time clause.

Here, Quan sortim per la porta means When we go out through the door or When we leave by the door.

It connects the action of leaving with the action in the main clause:

• first action: sortim
• second action: la meva mare apaga la llum

Catalan normally uses the definite article with possessives before many family members:

• la meva mare = my mother
• el meu pare = my father
• la meva germana = my sister

This is different from English, where you just say my mother.

A very important exception: in direct address, the article is usually dropped:

• Mare, vine aquí. = Mum/Mom, come here.

Per la porta means through the door, out the door, or by the door depending on context, but here the natural meaning is through/out the door.

Per often expresses movement through, along, by means of, or via something. With sortir, per la porta is a natural way to say that the exit happens through the door.

So:

• sortir per la porta = to go out through the door

A learner might expect something more literal from English, but in Catalan this is a normal expression.

Apaga is the 3rd person singular present tense of apagar, which means to turn off or to switch off.

So:

• apago = I turn off
• apagues = you turn off
• apaga = he/she turns off

In this sentence, the subject is la meva mare, so apaga means my mother turns off.

La llum literally means the light. In Catalan, this often refers to the room light or electric light in a general sense, just like English turn off the light.

So:

• apagar la llum = to turn off the light

If you were talking about several separate lights, you could say els llums, but here the singular is the normal choice.

Yes. Catalan allows some flexibility. You could also say:

La meva mare apaga la llum quan sortim per la porta.

That means the same thing. The version with Quan sortim per la porta first gives the time/background information before the main action.

Both are natural; the choice depends on what the speaker wants to emphasize.

The comma is normal here because the sentence begins with a subordinate time clause:

Quan sortim per la porta, la meva mare apaga la llum.

This works like English, where a comma is commonly used after an introductory clause: When we leave through the door, my mother turns off the light.

If the time clause comes after the main clause, the comma is often omitted: La meva mare apaga la llum quan sortim per la porta.

Grammatically, sortim only tells us that the subject is we. It does not specify exactly who is included. That has to come from context.

It could mean:

• the speaker and their family
• the speaker and another person
• the people in the house

So Catalan works like English here: we leave is grammatically clear, but the exact group depends on the situation.