Learn Hardcore Spanish (Spain): Cierra la puerta antes de que empiece la lluvia, por si acaso. - Close the door before the rain starts, just in case.

Because antes de que always introduces an action that is anticipated or not yet realized, so it triggers the subjunctive. Here, the rain hasn’t started yet, so we use the present subjunctive: empiece. Using the indicative (empieza) after antes de que is considered incorrect.

Present/imperative main clause → present subjunctive: Cierra la puerta antes de que empiece la lluvia.
Past main clause → imperfect subjunctive: Cerró la puerta antes de que empezara/empzase la lluvia.

Can I say antes de que empieza?

No. With antes de que, you must use the subjunctive: antes de que empiece. The indicative (empieza) doesn’t work after this conjunction.

Could I use empezara/ empezase here instead of empiece?

Not with an imperative in the main clause. With a present-time or imperative main clause, use the present subjunctive (empiece). The imperfect subjunctive (empezara/ empezase) is used when the main clause is in a past frame: Cerró la puerta antes de que empezara la lluvia.

Why antes de que and not just antes que?

Both forms exist, but antes de que is the safest and most standard choice before a verb clause in contemporary Spanish. Antes que is also accepted in many contexts, but learners are strongly advised to stick to antes de que to avoid queísmo/dequeísmo mistakes.

When do I use antes de + infinitive instead?

Use antes de + infinitive when both actions share the same subject:

Same subject: Cierra la puerta antes de irte. (You close; you leave.)
Different subjects: Cierra la puerta antes de que empiece la lluvia. (You close; the rain starts.)

Is antes de que empiece la lluvia natural, or should I say antes de que empiece a llover or antes de que llueva?

All are correct. In everyday speech, many speakers prefer using the verb llover:

…antes de que empiece a llover.
…antes de que llueva. Your version with the noun (la lluvia) is fine and a bit more formal/literal-sounding.

Do I need the comma before por si acaso?

When por si acaso is an afterthought/parenthetical, a comma is common and recommended: …, por si acaso.
If it introduces its own clause (por si + verb), you typically don’t use a comma: Cierra la puerta por si llueve.
If por si acaso comes first, use a comma after it: Por si acaso, cierra la puerta.

Do I need to add a verb after por si acaso?

Not necessarily. It can stand alone as an elliptical add-on meaning “just in case”: Cierra la puerta…, por si acaso.
If you spell it out, use por si + verb: Cierra la puerta por si (acaso) llueve.

Should I use the subjunctive after por si (acaso)?

Default: indicative → por si (acaso) llueve.
More remote/less likely: imperfect subjunctive → por si (acaso) lloviera/lloviese. Avoid present subjunctive here (por si llueva), which is not standard.

Is Cierra the right imperative form for Spain? What about formal or plural?

Yes. In Spain:

Tú (informal singular): Cierra la puerta.
Vosotros (informal plural): Cerrad la puerta.
Usted (formal singular): Cierre la puerta.
Ustedes (formal plural): Cierren la puerta. Negatives use the present subjunctive: No cierres / no cerréis / no cierre / no cierren.

Where do pronouns go with commands like this?

Affirmative: attach to the end; add a written accent if needed.
- Ciérrala. (Close it.)
- Cerradla.
- Ciérrela. / Ciérrenla.
Negative: place before the verb.
- No la cierres. / No la cerréis. / No la cierre. / No la cierren.

Why is it empiece and not empieze?

Because verbs ending in -zar change z → c before e/i in the subjunctive (and some other forms): empezar → empiece, empieces, empiece, empecemos, empecéis, empiecen.
Also note the stem change e → ie (except in nosotros/vosotros: empecemos, empecéis).

Why are both nouns feminine: la puerta, la lluvia?

Because their grammatical gender is feminine in Spanish. You must use the definite article la with them here. You can’t drop the article before lluvia in this structure; if you want to avoid the noun and the article, switch to the verb: antes de que llueva / empiece a llover.

Can I move por si acaso to the front?

Yes: Por si acaso, cierra la puerta antes de que empiece la lluvia. The meaning is the same; the comma helps mark it as a sentence-initial aside.

Any pronunciation tips for Spain (Castilian)?

Cierra: initial ci- is pronounced with the Castilian “th” sound: approx. “THYEH-rra” (roll the rr).
Empiece: -ce is also “th”: “em-PYEH-theh.”
Lluvia: in most of Spain, ll = y sound: “YOO-vyah.”