Os avós ficam felizes quando a Ana os visita.

Here, os is the definite article meaning the.

So:

• os avós = the grandparents

This os is not the same as the later os before visita, even though they look identical. At the beginning, it is an article; later, it is a pronoun.

This is a very common question because the accents matter a lot here.

• avó = grandmother
• avô = grandfather

Their plurals are:

• avós = grandmothers, or grandparents in a mixed group
• avôs = grandfathers

So in os avós, the meaning is the grandparents.
That is why avós is correct here, not avôs.

In Portuguese, ficar + adjective often means to become, to get, or to end up being in a certain state.

So:

• ficam felizes literally means something like become happy or get happy

In English, we often translate this more naturally as are happy in this kind of sentence:

• The grandparents are happy when Ana visits them

Why not the others?

• são felizes would mean they are happy people in general, as a permanent characteristic
• estão felizes would mean they are happy at that moment/state
• ficam felizes highlights the reaction caused by Ana’s visit

So ficam felizes is a very natural choice here.

Because the sentence is describing a habitual or general situation.

It means something like:

• whenever Ana visits them, the grandparents are happy

Portuguese often uses the present tense for this kind of repeated fact, just as English does.

So this sentence is not necessarily about one visit happening right now. It is more like a regular pattern.

In European Portuguese, it is very common to use the definite article before a person’s first name:

• a Ana
• o Pedro
• a Maria

This does not mean the Ana in English. It is just a normal Portuguese feature.

In Portugal, this sounds very natural in everyday speech and writing.
So a Ana os visita is completely normal.

This os is a direct object pronoun meaning them.

It refers back to os avós.

So instead of repeating the noun:

• quando a Ana visita os avós

Portuguese uses the pronoun:

• quando a Ana os visita

This is similar to English visits them.

This is especially important in European Portuguese.

Normally, in a simple affirmative sentence, object pronouns often come after the verb:

• A Ana visita-os. = Ana visits them.

But some words force the pronoun to come before the verb. This is called proclisis.

One of those trigger words is quando.

So after quando, you get:

• quando a Ana os visita

not

• quando a Ana visita-os

That is why the pronoun appears before visita here.

Because visitar takes a direct object, not an indirect one.

In Portuguese, you visit someone:

• visitar os avós
• visitar a Ana

So the correct object pronouns are:

• o / a / os / as

That is why the sentence has:

• os visita = visits them

Lhes is used for an indirect object, roughly to them, and it does not fit with visitar here.

Here quando means when, but in this kind of present-tense sentence it often has the sense of whenever.

So the idea is:

• whenever Ana visits them, the grandparents are happy

That habitual sense comes from the whole structure, especially the present tense in both clauses.

Yes. You could also say:

• Quando a Ana os visita, os avós ficam felizes.

The meaning is basically the same.

The difference is mainly one of focus:

• Os avós ficam felizes quando a Ana os visita.
  starts with the main idea: the grandparents’ reaction
• Quando a Ana os visita, os avós ficam felizes.
  starts with the time/condition clause

In both versions, os visita stays the same because quando still triggers the pronoun before the verb.