Quan recordo el meu avi i la meva àvia, estic feliç i no estic trista.

Quan means when.

In this sentence, it introduces a time clause: Quan recordo... = When I remember...

Because the rest of the sentence is also in the present tense, English will often translate it more naturally as whenever:

• Quan recordo el meu avi i la meva àvia...
  = Whenever I remember my grandfather and grandmother...

So it is not only about one single moment; it can describe a repeated feeling.

Recordo is the 1st person singular present indicative of recordar.

So:

• recordo = I remember
• recordes = you remember
• recorda = he/she remembers

The ending -o is a common I ending for many Catalan verbs in the present tense.

Catalan often leaves out subject pronouns when they are clear from the verb form.

So:

• recordo already means I remember
• estic already means I am

You could add jo for emphasis:

• Quan jo recordo...
• Jo estic feliç...

But in a neutral sentence, leaving jo out is more natural.

In Catalan, it is very normal to use the definite article before a possessive:

• el meu llibre = my book
• la meva mare = my mother
• el meu avi = my grandfather

This is different from English, which usually uses only my without the.

So el meu avi and la meva àvia are the standard Catalan pattern.

Because possessives in Catalan agree with the noun they describe.

Here:

• avi is masculine singular → el meu avi
• àvia is feminine singular → la meva àvia

So the possessive changes form:

• meu = masculine singular
• meva = feminine singular

The accent in àvia helps show the correct stress.

• àvia is stressed on the first syllable: À-vi-a

Without the accent, the reader might expect a different stress pattern.

Catalan accents are important because they often tell you:

• which syllable is stressed
• and sometimes the vowel quality too

Both ser and estar can translate as to be, but they are not used the same way.

Here, estic feliç is natural because it describes an emotional state connected to a situation:

• When I remember my grandparents, I feel happy

So estar is used for how someone feels at that moment or in that context.

Compare:

• Estic feliç = I feel happy / I am happy right now or in this situation
• Sóc feliç = I am a happy person / I am happy in life
  more like a general or lasting quality

Adjectives in Catalan often agree with gender.

In this sentence:

• trista is feminine singular
• so the speaker is understood to be female

A male speaker would say:

• no estic trist

But feliç is one of those adjectives that has the same singular form for both masculine and feminine:

• estic feliç (male speaker)
• estic feliç (female speaker)

So only trista/trist changes here.

Repeating estic is the normal and clear way to say it.

So:

• estic feliç i no estic trista

is more natural than trying to omit the second verb.

This is especially true because the second part is negative:

• no estic trista

Repeating the verb makes the contrast clear:

• I am happy and I am not sad

The whole sentence is in the present indicative:

• recordo
• estic
• estic

In Catalan, the present tense can describe:

• what is happening now
• a habit
• something that is generally true

Here it most likely means a repeated or general situation:

• Whenever I remember my grandfather and grandmother, I am happy and not sad

Because the sentence begins with a subordinate time clause:

• Quan recordo el meu avi i la meva àvia, ...

Then the main clause comes after it:

• estic feliç i no estic trista

The comma helps separate those two parts. This is similar to English:

• When I remember my grandparents, I am happy...

Yes. You could also put the quan clause later:

• Estic feliç i no estic trista quan recordo el meu avi i la meva àvia.

That is grammatically fine.

But the original order is very natural because it sets up the situation first:

• When I remember..., I am...

So the sentence starts with the condition or moment, then gives the emotional result.

Yes. That is another very common way to say I remember in Catalan, especially in everyday speech.

You could say:

• Quan me'n recordo del meu avi i de la meva àvia, estic feliç i no estic trista.

That structure is different:

• recordar + direct object → recordo el meu avi
• recordar-se'n de + noun → me'n recordo del meu avi

Both are correct, but the sentence you were given uses the simpler direct-object pattern:

• recordo el meu avi i la meva àvia