Learn Portuguese (Portugal): Organizamos a festa antes que ele chegue.

The phrase antes que (before) is a temporal conjunction that always triggers the subjunctive. Because his arrival hasn't happened yet at the time we organize the party, the action is treated as an anticipated or potential event rather than an established fact.

Could I use antes de instead of antes que in this sentence?

Yes, but the grammar would change. If you use the preposition antes de, you must use the infinitive verb form, making it antes de ele chegar. Both mean the exact same thing, though antes de is actually more common in everyday spoken Portuguese.

Why does chegue have a 'u' in it when the infinitive is chegar?

This is a spelling adjustment to keep the pronunciation consistent. In Portuguese, a 'g' followed by an 'e' makes a soft sound (like in 'gel'). To keep the hard 'g' sound from chegar, we must insert a 'u' before the subjunctive '-e' ending.

How do we know organizamos is in the present tense and not the past tense?

In European Portuguese, the 'we' form of regular '-ar' verbs distinguishes between the present and past tense using a written accent. The present tense is organizamos, while the past tense adds an acute accent to become organizámos. Since there is no accent in this sentence, it translates to 'we organize' rather than 'we organized'.