Quan arribi la meva tieta, li donarem el ram de flors.

Because after quan when Catalan refers to a future event that has not happened yet, it normally uses the present subjunctive, not the present indicative.

So:

• Quan arriba la meva tieta = when my aunt arrives / when my aunt usually arrives or a more factual, non-future reading
• Quan arribi la meva tieta = when my aunt arrives in the future

This is a very common pattern in Catalan:

• Quan tinguis temps, truca'm.
• Quan sigui gran...
• Quan acabem, marxarem.

Because Catalan usually avoids the future tense in this kind of time clause. After words like quan when the meaning is future, Catalan prefers:

• present subjunctive in the subordinate clause
• future or another appropriate tense in the main clause

So the natural pattern is:

• Quan arribi..., donarem...

not usually:

• Quan arribarà..., donarem...

This is different from English, which also avoids future in clauses like when she arrives, but Catalan goes one step further by using the subjunctive.

Arribi is the present subjunctive, 3rd person singular, of arribar.

Here is the present subjunctive of arribar:

• jo arribi
• tu arribis
• ell/ella/vostè arribi
• nosaltres arribem
• vosaltres arribeu
• ells/elles/vostès arribin

In this sentence, la meva tieta is she, so arribi is the correct form.

Li is the indirect object pronoun meaning to him / to her / to you (formal singular).

In this sentence, it means to her, referring to la meva tieta.

So:

• donarem el ram de flors = we will give the bouquet of flowers
• li donarem el ram de flors = we will give her the bouquet of flowers

Catalan uses these object pronouns very naturally and often more regularly than English does.

Because object pronouns in Catalan normally come before a conjugated verb.

So:

• li donarem
• la veig
• ens ajudaran

This is the normal position with a finite verb.
If the verb were an infinitive, gerund, or affirmative imperative, the position could change:

• donar-li
• donant-li
• dóna-li

But with donarem, the standard order is li donarem.

Because Catalan usually uses the definite article with possessives:

• el meu pare
• la meva mare
• el meu amic
• la nostra casa

So la meva tieta is completely normal Catalan.

English does not use an article before my, your, our, etc., but Catalan usually does.
There are some exceptions in special contexts, but as a general rule, article + possessive + noun is what learners should expect.

Because Catalan often uses the definite article when referring to something specific or identifiable in the situation.

El ram de flors means the bouquet of flowers.
It suggests a particular bouquet, probably the one already prepared or already understood from context.

If you wanted a more indefinite meaning, you might say:

• un ram de flors = a bouquet of flowers

So the sentence uses el because it is talking about a specific bouquet.

Literally, ram means bouquet or bunch, and de flors means of flowers.

So:

• ram = bouquet / bunch
• flors = flowers
• ram de flors = bouquet of flowers

This noun + de + noun structure is very common in Catalan:

• tassa de cafè = cup of coffee
• botella d’aigua = bottle of water
• taula de fusta = wooden table / table of wood

Yes. Catalan allows some flexibility.

For example, you could also say:

• Li donarem el ram de flors quan arribi la meva tieta.

This means the same thing. The difference is mostly one of focus or style:

• Quan arribi la meva tieta, ... puts the time condition first
• Li donarem el ram de flors quan arribi la meva tieta. starts with the main action

Both are natural.

The comma is natural and very common when the sentence begins with the quan clause:

• Quan arribi la meva tieta, li donarem el ram de flors.

It helps separate the introductory subordinate clause from the main clause.

If the order is reversed, the comma is often omitted:

• Li donarem el ram de flors quan arribi la meva tieta.

So the comma here is mainly about clear sentence structure and standard punctuation.

Only the verb arribi would change to match the new subject.

For example:

• Quan arribem nosaltres, ... = when we arrive
• Quan arribis tu, ... = when you arrive
• Quan arribin ells, ... = when they arrive

And if the main clause subject changed, donarem would change too:

• li donaré = I will give her
• li donaràs = you will give her
• li donaran = they will give her

So the sentence is a good example of normal agreement in Catalan:

• arribi agrees with la meva tieta
• donarem agrees with the implied subject we