Os meus amigos abraçam-se quando estão felizes.

Because amigos is a plural masculine noun, so the words that go with it must match:

• os = the for masculine plural nouns
• meus = my for masculine plural nouns
• amigos = friends / male friends / mixed group of friends

So:

• o meu amigo = my friend
• os meus amigos = my friends

If the group were all female, it would be:

• as minhas amigas

Because amigos is plural.

In Portuguese, possessives agree with the noun they describe:

• meu amigo = my friend
• meus amigos = my friends
• minha amiga = my female friend
• minhas amigas = my female friends

So meus is simply the masculine plural form of my.

Abraçam-se means they hug each other or they hug themselves/one another, depending on context. In this sentence, the natural meaning is they hug each other.

It is made of:

• abraçam = they hug
• -se = a pronoun that here gives the verb a reciprocal sense

So:

• Os meus amigos abraçam. = My friends hug.
• Os meus amigos abraçam-se. = My friends hug each other.

This is a very important point in European Portuguese.

In European Portuguese, it is very common to attach object pronouns to the end of the verb with a hyphen. This is called enclisis:

• abraçam-se
• levantam-se
• chamam-se

So abraçam-se is the normal form here.

In some situations, Portuguese requires the pronoun to come before the verb, but in this sentence there is no trigger forcing that. So abraçam-se is the expected structure in Portugal.

Not in this sentence as the normal European Portuguese form.

In European Portuguese, se abraçam would usually need a reason for the pronoun to move before the verb, such as a negative word, certain adverbs, or some subordinating words. For example:

• Não se abraçam. = They do not hug each other.
• Quando se abraçam, sorriem. = When they hug each other, they smile.

But in your sentence, the main clause is simply:

• Os meus amigos abraçam-se

So the attached form is the standard one.

The ç keeps the s sound before a, o, or u.

Without the cedilla:

• ca, co, cu usually sound like k

With the cedilla:

• ça, ço, çu sound like s

So:

• abraçam is pronounced with an s sound in that part, not a k sound.

This is just a spelling rule in Portuguese.

The mark in abraçam is actually not a tilde over the final vowel; the key point is that -am in this verb form marks the 3rd person plural of the present tense:

• eu abraço = I hug
• tu abraças = you hug
• ele/ela abraça = he/she hugs
• eles/elas abraçam = they hug

So abraçam means they hug.

In pronunciation, European Portuguese often reduces unstressed vowels, but grammatically the ending -am tells you the subject is they.

Because estar is often used for a state or condition, while ser is used more for a general characteristic.

Here, when they are happy means when they are feeling happy, so Portuguese uses estar:

• estão felizes = they are happy

If you said são felizes, it would usually mean something more like they are happy people or they are generally happy, as a more permanent or defining trait.

So in this sentence, estão is the natural choice.

Because the adjective must agree with the plural subject.

• ele está feliz = he is happy
• ela está feliz = she is happy
• eles estão felizes = they are happy
• elas estão felizes = they are happy

So feliz becomes felizes in the plural.

Quando means when.

It introduces the time clause:

• quando estão felizes = when they are happy

So the whole structure is:

• Os meus amigos abraçam-se = My friends hug each other
• quando estão felizes = when they are happy

Together: My friends hug each other when they are happy.

Because Portuguese often leaves out subject pronouns when the verb form already makes the subject clear.

• estão already tells you it is they / you plural
• In this sentence, the subject is already clearly os meus amigos

So Portuguese does not need to say eles here.

You could say quando eles estão felizes, but it is unnecessary and less natural unless you want emphasis or contrast.

Not necessarily.

In Portuguese, the masculine plural can refer to:

• a group of men
• a mixed group of men and women

So os meus amigos can mean:

• my male friends
• my friends, if the group includes at least one male
• sometimes just my friends in a general sense, depending on context

If the group is entirely female, then it must be:

• as minhas amigas

You would change all the matching words:

• O meu amigo abraça-se quando está feliz.

But be careful: with one person, abraça-se usually means he hugs himself, not he hugs someone else, because reciprocity needs more than one person.

So if you wanted:

• My friend hugs people when he is happy, you would need a different structure.

The plural in the original sentence is important because it makes the each other meaning natural.

Yes, the form abraçam-se strongly reflects normal European Portuguese usage.

In Brazilian Portuguese, people often prefer pronouns before the verb in everyday speech, so you may more often hear:

• Os meus amigos se abraçam quando estão felizes.

But in Portugal, abraçam-se is the standard and natural form here.

So if you are learning Portuguese from Portugal, this sentence is a very good model.