Je vais chauffer la soupe avant que Paul arrive.

Because aller + infinitive is a very common French way to talk about the near future.

• je vais chauffer = I’m going to heat / warm up
• literally, vais does come from aller = to go
• but in this structure, it works like English be going to

So here, je vais chauffer la soupe is the futur proche (near future), not a literal idea of physically going somewhere.

Both can be possible, but they are not exactly the same.

• chauffer = to heat
• réchauffer = to reheat / heat up again

If the soup is already made and is just being warmed again, many speakers would naturally say réchauffer la soupe.
If the focus is simply on making it hot, chauffer is still understandable and grammatical.

So this sentence is fine, but in everyday context, réchauffer might sometimes sound more natural.

French uses articles much more often than English.

In English, you can say:

• I’m heating soup
• I’m heating the soup

But in French, you usually need an article before a noun:

• la soupe = the soup

Here it suggests a specific soup already understood in the situation, even if English might leave out the.

Because the subject changes.

Use avant de + infinitive when the subject of both verbs is the same:

• Je vais chauffer la soupe avant de partir.
  • I’m going to heat the soup before leaving.
  • Same subject for both actions: I

Use avant que + clause when there is a different subject:

• Je vais chauffer la soupe avant que Paul arrive.
  • I heat the soup / am going to heat the soup before Paul arrives.
  • Different subjects: I and Paul

So avant que is required here because the second action has its own subject, Paul.

Because avant que normally requires the subjunctive in French.

So after avant que, French does not use the future tense here. It uses the subjunctive:

• avant que Paul arrive

Even though the meaning is future in English, French grammar chooses the subjunctive after this expression.

That is one of the biggest differences from English.

It is the present subjunctive form of arriver.

For arriver, the present subjunctive and the present indicative look the same in some persons:

• indicative: il arrive
• subjunctive: qu’il arrive

So the spelling is the same, but the grammar is different.
In this sentence, it is definitely subjunctive because it follows avant que.

You may also see:

• avant que Paul n’arrive

That ne is called the ne explétif. It does not make the sentence negative. It is just a stylistic or more formal feature that appears after certain expressions, including avant que.

So both are possible:

• avant que Paul arrive
• avant que Paul n’arrive

The version without ne is very common in modern spoken French.

Yes. That would also be grammatical.

The difference is mainly this:

• Je vais chauffer... = I’m going to heat...

  • very common in everyday spoken French
  • often feels more immediate or conversational

• Je chaufferai... = I will heat...

  • simple future
  • also correct, sometimes a bit more neutral or formal

Both can work with avant que Paul arrive.

Yes. The sentence presents Paul’s arrival as something still pending at that moment.

The order of events is:

1. I will heat the soup.
2. Paul will arrive after that.

So the speaker is talking about a point before Paul’s arrival.

Because French, like English, does not normally use a future tense after before in this kind of clause.

In English, we say:

• before Paul arrives not usually
• before Paul will arrive

French works similarly in meaning, but with its own grammar:

• avant que Paul arrive

So even though the idea is future, French does not use arrivera here. It uses the subjunctive after avant que.

Yes, it is natural and understandable.

A few notes:

• Je vais chauffer la soupe avant que Paul arrive is grammatical and normal.
• In many real-life situations, a native speaker might prefer réchauffer if the soup is already cooked.
• A more formal version could be avant que Paul n’arrive.

So the sentence is good French, with only a small possible vocabulary nuance depending on context.