Abans que la classe comenci, vull parlar amb la professora.

Because abans que normally triggers the subjunctive when you are talking about something that has not happened yet.

• comença = present indicative
• comenci = present subjunctive

In this sentence, the class has not started yet, so Catalan uses the subjunctive:

• Abans que la classe comenci...

This is a very common pattern:

• abans que + subjunctive
• després que + indicative/subjunctive depending on meaning and variety
• perquè + subjunctive in some purpose clauses, etc.

So here, comenci is exactly what you would expect.

Because abans que is used when it is followed by a full clause with its own subject and conjugated verb:

• abans que la classe comenci

Here you have:

• subject: la classe
• verb: comenci

By contrast, abans de is used before a noun or an infinitive:

• abans de la classe
• abans de començar

So:

• Abans que la classe comenci = before the class starts
• Abans de començar = before starting

In standard Catalan, no — the preferred form is abans que.

So the standard sentence is:

• Abans que la classe comenci...

Many learners ask this because Spanish has antes de que, but Catalan does not normally use that structure in the standard language.

A useful rule:

• abans de + infinitive/noun
• abans que + conjugated verb

Here que is a conjunction introducing the subordinate clause.

It links:

• Abans que... = before...
• la classe comenci = the class starts

So que is not really something you translate word-for-word every time. It is just part of the structure abans que.

Think of abans que as a unit meaning before when followed by a clause.

Because vull parlar means I want to speak, not I speak.

• vull = I want
• parlar = to speak

So:

• vull parlar = I want to speak
• parlo = I speak / I am speaking

Catalan often uses voler + infinitive just like English uses want + infinitive:

• Vull menjar = I want to eat
• Vull sortir = I want to leave
• Vull parlar = I want to speak

Because in Catalan, parlar amb algú is the normal way to say to speak with/to someone.

• parlar amb la professora = to speak with the teacher

English often uses to or with, but Catalan commonly uses amb in this structure.

So even though amb literally means with, in this context it is the natural choice.

Catalan uses definite articles very naturally, often more than English does.

Here:

• la classe = the class
• la professora = the teacher

This usually suggests a specific class and a specific teacher, understood from context.

Catalan often keeps the article where English might sometimes omit it in a more general sentence, but in this example both articles sound completely normal and expected.

Yes. Professora is feminine.

• el professor = the male teacher / professor
• la professora = the female teacher / professor

So this sentence tells you that the speaker wants to talk to a female teacher.

Yes. That is completely natural and means the same thing.

You can say either:

• Abans que la classe comenci, vull parlar amb la professora.
• Vull parlar amb la professora abans que la classe comenci.

The difference is mostly one of focus and style:

• putting Abans que la classe comenci first emphasizes the time condition
• putting Vull parlar amb la professora first emphasizes what you want to do

Also, when the time clause comes first, the comma is normal:

• Abans que la classe comenci, ...

Yes. Both are possible.

• abans que la classe comenci
• abans que comenci la classe

Catalan allows some flexibility in word order, especially in subordinate clauses.

The version with comenci la classe may sound a bit more fluid to some speakers, but la classe comenci is also correct. Both mean the same thing.

Usually no. If the second part does not need its own subject and conjugated verb, Catalan often prefers abans de + infinitive.

For example:

• Abans de parlar amb la professora, vull preparar-me.
• Abans de començar, necessito un moment.

But in your original sentence, the subject of the subordinate action is la classe, so a full clause is needed:

• Abans que la classe comenci...

That is why abans que is the right choice there.