Quan algú truca, apunta el seu número a l'agenda.

Here apunta is the affirmative imperative for tu of the verb apuntar.

• apuntar = to write down / note down
• (tu) apunta = write it down
• (vostè) apunti = write it down (formal singular)
• (tu) apuntes is not the affirmative command form

For regular -ar verbs, the affirmative tu command has the same form as the 3rd person singular present:

• ell/ella apunta
• (tu) apunta

So the sentence is giving an instruction: When someone calls, write down their number in the address book / contacts list.

Yes. By itself, apunta can mean either:

• he/she writes down
• write down! (informal singular command)

Catalan often has this kind of overlap. You understand which meaning is intended from context.

In this sentence, the most natural reading is the imperative: it sounds like an instruction or piece of advice.

Catalan uses the present tense very often for general truths, routines, and repeated situations.

So:

• Quan algú truca = When / Whenever someone calls

This is not just about one specific future moment. It means each time that happens.

Then the main clause gives the instruction:

• apunta el seu número... = write down their number...

So the sentence has the feel of a general rule or recommendation.

Quan means when.

In this sentence, it has the sense of whenever or every time that:

• Quan algú truca... = Whenever someone calls...

That is a very common use in Catalan.

Algú is an indefinite pronoun meaning someone / somebody / anyone.

A few useful points:

• It is singular
• It does not change for gender here
• The accent in algú shows the stressed final vowel

So Quan algú truca simply means When someone calls.

It can mean both, depending on context.

With phones, trucar usually means:

• to call
• to phone
• sometimes, by extension, to ring

So:

• Algú truca = Someone is calling / The phone is ringing because someone is calling

Also, if you say who receives the call, Catalan normally uses trucar a:

• Truco a la Maria = I’m calling Maria

In your sentence, there is no stated object, so it just means someone calls.

Not necessarily. Catalan often leaves that idea implicit if it is obvious from context.

So:

• Quan algú truca... = When someone calls...

In many situations, that naturally means when someone calls you / your phone.

If you want to make it explicit, you could say:

• Quan algú et truca... = When someone calls you...

But the version without et is perfectly normal in a general instruction like this.

Because the possessive agrees with the thing possessed, not with the person who has it.

Here:

• número is masculine singular
• so you get el seu número

Even though algú could refer to any person, the possessive form matches número, not the caller’s gender.

Also, seu/seva/seus/seves can mean different things depending on context:

• his
• her
• their
• your (formal)
• its

Here it corresponds to English their in a gender-neutral sense.

Here apuntar means to write down, to note down, or to record.

So:

• apunta el seu número = write down their number

In this context, it does not mean to point.

Depending on context, apuntar can also mean things like:

• to aim
• to point toward
• to sign up

But here the meaning is clearly note down / record.

A l'agenda means in the agenda / in the address book / in the contacts list, depending on context.

Literally:

• a = to / in
• l'agenda = the agenda

But here, because we are talking about a phone number, agenda is best understood as:

• address book
• contact list
• phone contacts

So the idea is: write down the number in your contacts/address book.

Because in Catalan, the singular article la is usually shortened to l' before a vowel or mute h.

So:

• la agenda → l'agenda

And with the preposition a, you get:

• a + l'agenda = a l'agenda

This is completely normal Catalan spelling.

It can mean several related things, and context decides.

Agenda can refer to:

• a diary/planner
• a schedule
• an address book / contacts list

In this sentence, because it mentions a phone number, the most natural meaning is address book or contacts list, not schedule.

Because Quan algú truca is a subordinate clause placed before the main clause.

Catalan normally uses a comma in this structure, just like English often does:

• When someone calls, write down their number.

So the comma helps separate:

• the time clause: Quan algú truca
• the main instruction: apunta el seu número a l'agenda

It is the standard punctuation here.