A não ser que haja trânsito, vou passar na padaria antes da aula.

Yes. A não ser que is a very common fixed expression meaning unless.

It introduces an exception to what the speaker is planning or expecting:

• A não ser que haja trânsito... = Unless there’s traffic...

A useful thing to remember is that a não ser que is normally followed by the subjunctive.

Because a não ser que requires the subjunctive mood, and haja is the present subjunctive form of haver.

Compare:

• há trânsito = there is traffic
• a não ser que haja trânsito = unless there is traffic

The speaker is not stating a fact here; they are talking about a possible condition, so Portuguese uses the subjunctive.

In this sentence, haver is being used in an impersonal way, similar to there is / there are in English.

So:

• há trânsito = there is traffic
• haja trânsito = there be traffic / there is traffic, in a subjunctive context

With things like trânsito, problemas, dificuldades, tempo, haver is very natural in standard Portuguese.

In everyday spoken Portuguese, ter often replaces haver in many contexts, especially for there is / there are. But with trânsito, haver trânsito is a very natural and standard choice.

So this sentence sounds very good as it is:

• A não ser que haja trânsito...

You may also hear:

• Se não houver trânsito...

That is also very natural.

Here, trânsito means traffic in the road/transport sense.

Depending on context, it can refer to:

• traffic in general
• heavy traffic
• traffic congestion

So haja trânsito does not necessarily mean a complete traffic jam; it means there is enough traffic to affect the plan.

Not exactly. In this context, passar em or passar por a place often means to stop by, to drop in, or to go there briefly on the way.

So:

• vou passar na padaria = I’m going to stop by the bakery

It suggests a quick visit, probably to buy something before going somewhere else.

Because na is the contraction of:

• em + a = na

With passar em algum lugar (to stop by somewhere), Portuguese uses em. Since padaria is feminine and normally takes the definite article a, you get:

• passar em a padaria → passar na padaria

So na padaria is completely normal grammar.

Because that would go with a different verb pattern.

• ir à padaria = to go to the bakery
• passar na padaria = to stop by the bakery

So the preposition depends on the verb:

• ir a → à
• passar em → na

That is why the sentence uses na padaria, not à padaria.

Vou passar is the very common near future / planned future structure:

• ir + infinitive

It often sounds more natural in conversation than the simple future.

So these are all possible, but with different feels:

• Vou passar na padaria = I’m going to stop by the bakery
• Passo na padaria = I’ll stop by the bakery / I stop by the bakery, depending on context
• Passarei na padaria = I will stop by the bakery, a bit more formal or written

In everyday spoken Portuguese, vou passar is extremely common.

Because antes is normally followed by de, and da is the contraction of:

• de + a = da

So:

• antes de a aula → antes da aula

This means before class or before the lesson.

Aula usually means class, lesson, or lecture, depending on context.

So antes da aula could mean:

• before school class
• before a private lesson
• before a university lecture

The exact English word depends on the situation, but the Portuguese structure stays the same.

Because the sentence begins with a subordinate clause:

• A não ser que haja trânsito, ...

In Portuguese, when a conditional or introductory clause comes first, it is very common to separate it from the main clause with a comma.

So the structure is:

• A não ser que haja trânsito, = introductory condition
• vou passar na padaria antes da aula. = main statement

The comma helps the sentence read clearly.

Yes, absolutely. That version is also natural.

There is a small nuance:

• A não ser que haja trânsito... = the speaker presents traffic as the main exception to the plan
• Se não houver trânsito... = a more neutral if-clause

So both are correct, but a não ser que highlights the idea of unless / except if more strongly.

It works in both, but it fits European Portuguese very naturally.

In both varieties, people would understand it perfectly. The main differences would be pronunciation and some style preferences, not the basic grammar here.

For a learner of Portuguese from Portugal, this sentence is fully natural and idiomatic.