Elas formam um casal tranquilo e têm muita confiança uma na outra.

Elas means they for an all-female group. In this sentence, it shows that the couple is made up of two women.

Portuguese usually marks gender in third-person plural pronouns:

• eles = they (all masculine or mixed group)
• elas = they (all feminine)

So elas is used because both people are female.

Formam is the third-person plural form of formar.

Here, formar um casal means to make up a couple or to form a couple. It is a natural way to say that two people are a couple together.

So:

• elas formam um casal = they make up a couple / they are a couple

It sounds a little more formal or descriptive than simply saying elas são um casal.

Because casal is a masculine noun in Portuguese. Grammatical gender does not always match the real-life gender of the people involved.

So you say:

• um casal
• o casal
• um casal tranquilo

even if the couple is made up of two women.

This is very common in Portuguese: the grammatical gender belongs to the noun itself, not necessarily to the people it refers to.

Because tranquilo agrees with casal, not with elas.

The structure is:

• um casal tranquilo

Since casal is masculine singular, the adjective must also be masculine singular:

• casal tranquilo

If the adjective described elas, then it would be feminine plural:

• Elas são tranquilas.

But in your sentence, the adjective is attached to casal, not directly to elas.

Here tranquilo means something like:

• calm
• peaceful
• easygoing

When used with casal, it suggests that they are a calm, stable, low-drama couple.

Depending on context, it could describe the relationship dynamic rather than each individual person separately.

The accent distinguishes third-person plural from third-person singular in the verb ter:

• ele/ela tem = he/she has
• eles/elas têm = they have

So in your sentence:

• elas têm = they have

This accent is very important in writing.

Confiança means trust or confidence.

In this sentence, ter confiança em alguém means to trust someone or to have trust in someone.

So:

• têm muita confiança = they have a lot of trust
• in context: they trust each other a lot

Portuguese often uses a noun phrase like ter confiança where English might prefer a verb like trust.

Portuguese can express this idea in more than one natural way.

Two common options are:

• ter confiança em alguém = to have trust in someone
• confiar em alguém = to trust someone

So this sentence could also be:

• Elas confiam muito uma na outra.

That means nearly the same thing.

The version with têm muita confiança feels a bit more noun-based, while confiam is more direct and verb-based.

Uma na outra is a reciprocal expression meaning in each other or one another, depending on how it works in English.

In this sentence:

• têm muita confiança uma na outra = they have a lot of trust in each other

The pattern changes according to gender and number:

• um no outro = one in the other / each other, masculine
• uma na outra = one in the other / each other, feminine

Because the sentence is about two women, Portuguese uses the feminine form:

• uma na outra

Because Portuguese normally contracts em + a into na.

So:

• em + a outra → na outra

Other common contractions are:

• em + o → no
• em + os → nos
• em + as → nas

So uma na outra literally comes from uma em a outra, but the contracted form na is the correct natural version.

Yes, you could. It is grammatical and natural.

Compare:

• Elas formam um casal tranquilo.
• Elas são um casal tranquilo.

Both mean roughly that they are a calm couple.

The difference is slight:

• formam um casal emphasizes the two people together as making up that couple
• são um casal is the more direct they are a couple

Both are good, but formam can sound a little more descriptive or literary.

No. It can be used whenever two female people do something to each other or in relation to each other, depending on the verb.

For example:

• Elas confiam uma na outra. = They trust each other.
• Elas pensam uma na outra. = They think about each other.
  (Though with this verb, other structures may be more common depending on context.)

So the expression is not inherently romantic. It just marks reciprocity between two feminine referents.

A simple breakdown is:

• Elas = subject
• formam = verb
• um casal tranquilo = object/complement
• e = and
• têm = verb
• muita confiança = object
• uma na outra = reciprocal complement, meaning in each other

So the sentence has two coordinated parts:

1. Elas formam um casal tranquilo
2. e têm muita confiança uma na outra

That makes it easier to read and understand piece by piece.