Si vols enviar alguna cosa a la meva àvia, posa bé l'adreça al paquet.

Si means if and introduces a condition.

In this sentence, Si vols enviar... means If you want to send...

A useful thing to notice is that Catalan uses the present tense after si for normal real conditions:

• Si tens temps, vine. = If you have time, come.
• Si vols, t'ajudo. = If you want, I’ll help you.

So si vols is the normal, natural way to say if you want.

Because this is a straightforward, real condition, Catalan uses the present indicative:

• si vols = if you want

The subjunctive is not used here.

That is very similar to Spanish and different from what English learners sometimes expect when they hear that Romance languages use the subjunctive a lot. After si in ordinary real conditions, Catalan normally takes the indicative:

• Si pots, truca’m. = If you can, call me.
• Si ve, parlarem. = If he/she comes, we’ll talk.

So si vulguis would be wrong here.

Catalan often drops subject pronouns because the verb ending already shows who the subject is.

Here:

• vols clearly means you want (informal singular)
• posa is the you singular imperative: put / write

So Catalan does not need tu unless the speaker wants emphasis or contrast:

• Si tu vols, ho faig jo. = If you want, I’ll do it.

In the sentence you gave, leaving out tu is the most natural choice.

After voler (to want), Catalan normally uses an infinitive:

• vols enviar = you want to send
• vull menjar = I want to eat
• volem marxar = we want to leave

Then alguna cosa means something.

So the structure is:

• vols = you want
• enviar = to send
• alguna cosa = something

Literally: if you want to send something...

Catalan often expresses something as alguna cosa, literally some thing.

This is completely normal and very common.

Examples:

• He vist alguna cosa estranya. = I saw something strange.
• Vols menjar alguna cosa? = Do you want to eat something?

In more informal speech, you may also hear algo in some varieties, but standard Catalan strongly prefers alguna cosa.

The a marks the recipient of what is being sent.

So:

• enviar alguna cosa a la meva àvia = to send something to my grandmother

That a is very common with people, especially when they are the indirect object or destination/recipient.

Also notice la meva àvia:

• àvia = grandmother
• meva = my
• la = the

Catalan usually uses the definite article with possessives:

• la meva mare = my mother
• el meu pare = my father
• la seva casa = his/her house

This often feels unusual to English speakers, because English says just my grandmother, not the my grandmother.

That is the normal word order in Catalan with possessives:

• la meva àvia
• el meu amic
• les nostres filles

So the pattern is usually:

article + possessive + noun

Examples:

• la meva germana = my sister
• el teu cotxe = your car
• els seus llibres = his/her/their books

English does not use an article here, but Catalan usually does.

Posa is the informal singular imperative of posar.

Here it is being used as a command to one person whom the speaker addresses as tu:

• posa bé l'adreça al paquet = put/write the address properly on the package

Imperative forms change depending on who you are speaking to:

• posa = command to tu
• posi = command to vostè (formal singular)
• poseu = command to vosaltres (plural)

So if you were speaking politely to one person, you would say:

• Si vol enviar alguna cosa a la meva àvia, posi bé l'adreça al paquet.

Here bé means properly, correctly, or clearly enough.

So posa bé l'adreça means something like:

• write the address properly
• make sure the address is written correctly
• put the address on clearly

It is not about feeling well; it is an adverb modifying the verb posa.

Compare:

• Ho has fet bé. = You did it well.
• Escriu bé el nom. = Write the name correctly.
• Posa bé l’adreça. = Put/write the address properly.

Because la contracts by elision before a vowel.

So:

• la + adreça becomes l'adreça

This is very common in Catalan:

• l'àvia = the grandmother
• l'escola = the school
• l'amiga = the female friend

The apostrophe shows that the vowel of la has dropped.

In this context, adreça means address, especially a mailing or postal address.

That is the normal word when talking about where a package or letter should go.

Examples:

• Escriu l’adreça. = Write the address.
• No tinc la seva adreça. = I don’t have his/her address.

A learner might wonder about direcció, because it can also mean direction or sometimes address in some contexts, but for postal purposes adreça is the standard and most natural choice.

Al is the contraction of a + el:

• a el paquet → al paquet

This is just like:

• al cotxe = to/on the car
• al carrer = in the street / onto the street, depending on context

Since paquet is masculine singular, it uses el, and with a that becomes al.

So:

• posa bé l'adreça al paquet

means putting or writing the address on the package.

Catalan often uses a where English uses on, in, or another preposition, depending on the context.

With labels, writing, and placement on an object, a can be perfectly natural:

• al paquet = on the package
• a la porta = on the door / at the door, depending on context

So you should not always translate prepositions word-for-word from English. In this sentence, al paquet is the normal Catalan way to express where the address should appear.

The verb forms would change.

Informal singular

• Si vols enviar alguna cosa a la meva àvia, posa bé l'adreça al paquet.

Formal singular

• Si vol enviar alguna cosa a la meva àvia, posi bé l'adreça al paquet.

Informal plural

• Si voleu enviar alguna cosa a la meva àvia, poseu bé l'adreça al paquet.

This is useful because Catalan marks politeness and number clearly in the verb forms.

Àvia means grandmother and the accent mark shows the stressed syllable.

It is pronounced roughly like AH-vee-uh, with the stress on the first syllable.

The grave accent in à tells you that the stress falls there and also distinguishes spelling.

A few pronunciation notes for the whole sentence:

• si = roughly see
• vols = roughly bols with a Catalan o
• àvia = AH-vee-uh
• bé = beh with stress
• adreça = roughly uh-DREH-suh
• paquet = roughly puh-KET

Exact pronunciation depends a bit on dialect, but those approximations are a good start.