Se eu tivesse ido à dentista na semana passada, não teria passado a noite com o lábio inchado.

This is a past unreal conditional, often called the third conditional in English-style grammar explanations.

Its basic pattern in Portuguese is:

Se + past perfect subjunctive, conditional perfect

So here we have:

• Se eu tivesse ido...
• não teria passado...

It describes something that did not happen in the past, and an imagined result that also did not happen the way the speaker wishes it had.

Tivesse ido is the past perfect subjunctive of ir in a compound form.

It is built with:

• tivesse = imperfect subjunctive of ter
• ido = past participle of ir

So:

• ter ido = to have gone
• tivesse ido = had gone

Portuguese uses this form after se when talking about a hypothetical past situation:

• Se eu tivesse estudado...
• Se ela tivesse chegado mais cedo...

In this sentence, tivesse ido shows an unreal past action: the speaker did not go to the dentist.

Because after se in this kind of conditional sentence, Portuguese normally uses the subjunctive, not the conditional.

So the standard pattern is:

• Se eu tivesse ido... , teria...
• not Se eu teria ido...

This is very similar to English:

• If I had gone... , I would have...
• not If I would have gone...

For many English speakers, this is a very common mistake at first.

Teria passado is the conditional perfect.

It is formed with:

• teria = conditional of ter
• passado = past participle of passar

So:

• teria passado = would have spent here

In the sentence, it expresses the imagined result of the unreal condition:

• Se eu tivesse ido...
• não teria passado a noite...

In other words, because the speaker did not go to the dentist, they did end up spending the night with a swollen lip.

The à is a crase, which happens when two a sounds combine:

• the preposition a required by ir
• the feminine article a before dentista

So:

• ir a + a dentista = ir à dentista

That is why you see à with a grave accent.

Compare:

• ir ao médico = go to the doctor
• ir à dentista = go to the dentist

If the professional were referred to with a masculine article, you would get ao, not à.

Because dentista is one of those Portuguese nouns that usually keep the same form for both masculine and feminine.

The article shows the gender:

• o dentista = the male dentist
• a dentista = the female dentist

So in this sentence, à dentista strongly suggests that the dentist is a woman.

This is common with some profession words in Portuguese.

Ao dentista can be correct, but it changes the gender reference:

• à dentista = to the female dentist
• ao dentista = to the male dentist, or sometimes a masculine generic reference

As for no dentista, in informal Brazilian speech some people do say things like ir no médico or ir no dentista, especially in conversation. But in more standard grammar, ir ao/à dentista is the expected form here.

So for learners, the safest choice is:

• ir ao dentista
• ir à dentista

depending on the gender/article you want.

Yes, both can be natural.

• na semana passada = literally in the last week / in the past week
• semana passada = last week

Here, na is the contraction of:

• em + a = na

So:

• na semana passada = in the week that passed / last week

In Brazilian Portuguese, both na semana passada and semana passada are common. The version with na can sound a little more anchored inside a time frame, while semana passada is often a bit simpler and more direct.

Because passar a noite is a very common expression meaning to spend the night.

So:

• passar a noite estudando = spend the night studying
• passar a noite acordado = spend the night awake
• passar a noite com o lábio inchado = spend the night with a swollen lip

Even though passar often means to pass, it also has the meaning to spend time in many contexts.

Com + noun + adjective is a very common Portuguese way to describe someone being in a certain physical condition or state.

So:

• com febre = with a fever
• com dor de cabeça = with a headache
• com o braço quebrado = with a broken arm
• com o lábio inchado = with a swollen lip

Here, inchado describes the condition of o lábio.

It is very natural in Portuguese to express the result this way.

Yes, plural is possible too.

• com o lábio inchado = with the lip swollen
• com os lábios inchados = with swollen lips

The singular suggests one lip in particular, or it can simply sound more focused and concrete. The plural would also be perfectly understandable if that is what the speaker means physically.

So this is not really a grammar problem; it is more about what exact image the speaker wants to give.

They come from the same verb family, but they are doing different jobs here.

1. teria passado

   • here passado is the past participle of passar
   • it is part of the verb phrase
   • meaning: would have spent

2. semana passada

   • here passada works like an adjective
   • meaning: last or literally past

So although they look similar, they are not functioning the same way in the sentence.

Not with exactly the same meaning.

• Se eu tivesse ido à dentista... means if I had gone to the dentist

• it refers to a specific past event that did not happen

• Se eu fosse à dentista... usually means if I went to the dentist or if I were to go to the dentist
• it sounds less tied to one completed past event

Because the sentence mentions na semana passada and describes a result that happened afterward, tivesse ido is the best choice. It clearly marks a missed action in the past.