Learn Hardcore Portuguese (Portugal): Eu sou o único passageiro, por isso sento-me à frente ao lado da motorista. - I am the only passenger, so I sit in the front next to the driver.

ser is used for permanent or defining characteristics, identity, profession, origin, etc.
estar is used for temporary states, locations, feelings, ongoing actions, etc.

In Eu sou o único passageiro, being the only passenger is presented as a defining fact of the situation, not a temporary feeling or mood. It’s like saying in English: I am the only passenger (a role/identity in that context), so ser is the correct choice.

If you said Eu estou o único passageiro, it would be ungrammatical in Portuguese.

Can I drop Eu and just say Sou o único passageiro?

Yes. In Portuguese, subject pronouns (like eu, tu, ele) are often omitted because the verb ending already shows who the subject is.

Eu sou o único passageiro = Sou o único passageiro

Both are correct. Using Eu adds a bit of emphasis, like I am the only passenger (maybe contrasting with others).
Without Eu, it sounds a bit more neutral.

Why do we say o único passageiro and not just único passageiro?

Portuguese normally uses a definite article (o/a/os/as) before nouns much more than English does.

o único passageiro literally: the only passenger
único passageiro without o sounds incomplete or ungrammatical here.

In contexts like this, a role or identity is nearly always introduced with a definite article:

Eu sou o professor. – I’m the teacher.
Ela é a diretora. – She is the director.
Eu sou o único passageiro. – I’m the only passenger.

Leaving out o would be very odd for a native speaker.

Why is it único passageiro and not única passageira? What changes if the speaker is a woman?

Adjectives and many nouns in Portuguese agree in gender and number.

passageiro – male passenger
passageira – female passenger
único – masculine singular
única – feminine singular

In the sentence:

Eu sou o único passageiro – the speaker is male.
If the speaker is a woman, it should be:
Eu sou a única passageira.

So you change:

the article: o → a
the adjective: único → única
the noun: passageiro → passageira

What exactly does por isso mean? Is it the same as porque or então?

por isso literally means for that (reason) and functions like so / therefore / that’s why.

Eu sou o único passageiro, por isso sento-me à frente…
→ I’m the only passenger, so I sit in the front…

Compare:

porque = because (introduces the reason)
- Sento-me à frente porque sou o único passageiro.
  – I sit in the front because I’m the only passenger.
por isso = so / therefore (introduces the consequence)
- Sou o único passageiro, por isso sento-me à frente.
  – I’m the only passenger, so I sit in the front.
então can also mean so / then, and is very common in speech. In European Portuguese, por isso is slightly more neutral/formal, então a bit more colloquial. In this sentence you could also say:
- …sou o único passageiro, então sento-me à frente… (more informal).

Why is sento-me reflexive? What’s the difference between sento-me and sento?

The verb is sentar-se (to sit down / to sit oneself), and it’s reflexive in Portuguese in this sense.

eu sento-me – I sit (down) / I take a seat
eu sento alguém – I seat someone else / I make someone sit

So:

Sento-me à frente – I sit (myself) in the front.
If you just said Sento à frente, it would sound like you’re seating someone else: I seat (someone) in the front, but with the object missing.

In normal contexts when talking about where you sit, you use the reflexive: sentar-se → sento-me.

Why is the pronoun attached to the verb as sento-me, and not written before, like me sento?

This is a key difference between European and Brazilian Portuguese.

In European Portuguese, the default in affirmative main clauses is to put the clitic pronoun after the verb with a hyphen:
- sento-me, levanto-me, chamo-me João.
In Brazilian Portuguese, the common spoken pattern is:
- me sento, me levanto, me chamo João (pronoun before the verb).

So in Portugal, sento-me is the natural and correct form here.
Me sento would sound clearly Brazilian.

You move the pronoun in European Portuguese (to before the verb) mainly after certain words like não, já, que, etc., but in a simple affirmative sentence like this, it goes after the verb.

What does à frente mean exactly, and how is it different from na frente or em frente?

All three relate to “in front / at the front”, but they’re used a bit differently.

à frente
- Literally: at the front / up front / ahead
- Common for being in the front part of something (car, bus, queue) or ahead in position.
- In this sentence, sento-me à frente = I sit in the front (of the vehicle).
na frente (de)
- Literally: in the front (of)
- Often means in front of (something) as a separate object or place:
  - Ele está na frente de casa. – He is in front of the house.
- It can also mean in the front part of something, but for a car seat position, à frente is more idiomatic in European Portuguese.
em frente (de)
- Often means directly opposite / across from / in front of:
  - O supermercado fica em frente à escola. – The supermarket is opposite the school.

For where you sit in a car, bus, etc., sentar-se à frente is the standard choice.

How is ao lado da motorista formed? What does ao and da mean here?

This part is built with preposition + article contractions, which are very common in Portuguese.

ao lado de = next to / beside / by the side of
- ao = a + o (to/at + the, masculine singular)
- lado = side
- de = of
da motorista
- da = de + a (of + the, feminine singular)
- motorista = driver

So ao lado da motorista literally breaks down as:

ao lado – at the side / by the side
da motorista – of the (female) driver

Together: next to the driver.

Why is it da motorista and not do motorista? How does gender work with motorista?

Nouns ending in -ista (like motorista, jornalista, pianista) usually have the same form for both masculine and feminine. The gender is shown by the article and any adjectives, not by the noun ending.

o motorista – the male driver
a motorista – the female driver

Then, with de + o / de + a, you get:

do motorista = de + o motorista – of the male driver
da motorista = de + a motorista – of the female driver

In the sentence, da motorista tells us the driver is female.

Can I change the word order and say sento-me ao lado da motorista à frente?

Yes, that’s also grammatically correct.

sento-me à frente ao lado da motorista
sento-me ao lado da motorista à frente

Both would be understood as I sit in the front, next to the driver.

Native speakers often say (and write) it with a small pause or comma:

…sento-me à frente, ao lado da motorista.

That version is probably the clearest: first you say where (in the front), then exactly where (next to the driver).

Would this sentence be different in Brazilian Portuguese?

The vocabulary is fine for Brazil too, but Brazilians would usually change the pronoun position and might tweak a detail or two. For example:

Eu sou o único passageiro, por isso me sento na frente, ao lado da motorista.

Differences:

me sento (Brazil) vs sento-me (Portugal)
na frente is more common in Brazil for in the front (seat), though à frente is also understood.
Everything else (including por isso, ao lado da motorista) is acceptable in both varieties.

So your original sentence is clearly European Portuguese; a Brazilian would tend to say it slightly differently.