Se chover, eu vou trocar de tênis antes que a aula comece.

Because after se when you mean a possible future condition, Portuguese normally uses the future subjunctive.

• Se chover = If it rains
• Se eu tiver tempo = If I have time
• Se eles chegarem cedo = If they arrive early

With many verbs, the future subjunctive looks the same as the infinitive, so chover here may look like an infinitive, but its job in the sentence is future subjunctive.

Because antes que is a conjunction that normally requires the subjunctive.

So:

• antes que a aula comece = before class starts

The idea is that the class has not started yet at that moment, so Portuguese treats it as something anticipated or not yet realized, which is a typical subjunctive situation.

Compare:

• Espero que ele venha.
• É importante que você estude.
• Antes que a aula comece.

All of those use the subjunctive after a trigger expression.

Yes, but they are different kinds of subjunctive.

• chover after se = future subjunctive
• comece after antes que = present subjunctive

Portuguese uses different subjunctive tenses depending on the structure:

• se + future condition → often future subjunctive
• antes que → usually present subjunctive

So both are subjunctive, but they are not the same tense.

Because in Brazilian Portuguese, ir + infinitive is extremely common for the future, especially in everyday speech.

• eu vou trocar = natural, common spoken Brazilian Portuguese
• eu trocarei = correct, but more formal or literary

So eu vou trocar de tênis sounds very natural in conversation.

It means to change sneakers/shoes or to switch to a different pair of sneakers.

In this sentence, the speaker is saying they will put on different footwear before class starts if it rains.

It does not mean exchange sneakers with someone or return them at a store. Here trocar means change in the sense of what you are wearing.

Because Portuguese often uses trocar de + clothing/item when talking about changing what you are wearing.

Very common patterns:

• trocar de roupa = change clothes
• trocar de camisa = change shirts / change your shirt
• trocar de sapato = change shoes
• trocar de tênis = change sneakers

If you say trocar o tênis, that can sound more like replacing that specific pair.
Trocar de tênis focuses more on changing what you have on.

It is treated as a singular noun here, even though in English sneakers is usually plural.

In Brazilian Portuguese, tênis often refers to a pair of sneakers/shoes as one item:

• o tênis = the sneaker pair / the tennis shoe(s)
• os tênis = the sneakers / multiple pairs, depending on context

So trocar de tênis is completely normal and does not sound incomplete.

Because Portuguese uses definite articles more often than English.

Here a aula means the class — the specific class both speaker and listener can identify from context.

English often says just before class starts, but Portuguese naturally says:

• antes que a aula comece

Leaving out the article would sound less natural in this sentence.

Yes. That is very natural.

Portuguese often drops subject pronouns when the verb already makes the subject clear:

• (Eu) vou trocar
• (Nós) vamos sair
• (Ela) chegou cedo

So both of these are fine:

• Se chover, eu vou trocar de tênis antes que a aula comece.
• Se chover, vou trocar de tênis antes que a aula comece.

The version without eu is often more natural in normal speech.

Yes. That is also possible.

These are both natural:

• antes que a aula comece
• antes de a aula começar

They mean basically the same thing here.

A small difference:

• antes que + subjunctive is a very standard finite clause
• antes de + infinitive is an infinitive construction

In everyday Brazilian Portuguese, you may also hear antes da aula começar, especially in speech, although more careful grammar traditionally prefers antes de a aula começar when the subject a aula is expressed.

Not exactly. They express different ideas.

• se chover = if it rains
  This means rain is only a possibility.
• quando chover = when it rains
  This suggests the speaker assumes the rain will happen.

So in your sentence, se chover is the better choice if the speaker is talking about a possible condition.

Because Se chover is an introductory conditional clause placed before the main clause.

So the comma helps separate:

• Se chover, = the condition
• eu vou trocar de tênis... = the main action

This punctuation is standard and very common in Portuguese.