Learn Hardcore Spanish (Spain): Cierra la puerta, así no entra el polvo. - Close the door, that way no dust comes in.

Yes. Cierra is the affirmative informal singular command (tú) of cerrar. It comes from the 3rd-person singular present indicative (cierra) and keeps the stem change (e > ie). The formal singular command would be cierre; the plural forms are covered below.

Why don’t we say tú? Where is the subject?

Spanish normally drops subject pronouns because the verb ending already shows the person. Cierra clearly implies tú, so adding tú is unnecessary (though you can say Tú, cierra la puerta for emphasis).

What are the other imperative forms for this verb in Spain (including negatives)?

Tú: cierra; negative: no cierres
Usted: cierre; negative: no cierre
Vosotros: cerrad; negative: no cerréis
Ustedes: cierren; negative: no cierren

How do I say “Close it” referring to the door? Where do pronouns go?

Affirmative: attach pronouns to the verb: Ciérrala (la = la puerta).
Negative: place pronouns before the verb: No la cierres. This pattern (attach in affirmative; place before in negative) is general for commands.

Why does Ciérrala have an accent?

Adding a pronoun shifts the stress. Writing cierrala would wrongly stress the second-to-last syllable. The accent in Ciérrala keeps the original stress (CIÉ-rra-la). Same with Ciérrela, Ciérrenla, etc.

What does así do here? Why not así que or entonces?

Here así means “in this way/like this,” expressing manner and a practical result: “Do X; this way Y doesn’t happen.”

así que is a conjunction meaning “so/therefore” and sounds off after a command here.
entonces mainly marks sequence (“then”) and isn’t used to express “this way/by doing this” in this structure.
So ..., así no entra... is the natural choice.

Could I say ... para que no entre el polvo instead? Is there a nuance?

Yes: Cierra la puerta para que no entre el polvo.

para que + subjunctive (entre) states purpose (“so that”).
..., así no entra... states a result/method (“this way it doesn’t come in”) and is more colloquial. Both are fine; the focus just shifts slightly.

Why is it no entra (present) and not no entrará (future)?

Spanish often uses the present to talk about immediate, predictable results. Así no entra sounds natural and general. Así no entrará is also possible, but it sounds more like a prediction; it’s less common in everyday instructions.

Is the comma before así correct? Could I use a semicolon or a dash?

Yes, the comma is normal in Spanish to add this brief result clause. A semicolon or dash (—) can also be used to mark the consequence more clearly:

Cierra la puerta; así no entra el polvo.
Cierra la puerta — así no entra el polvo.

Can así go first?

Yes: Así no entra el polvo; cierra la puerta. Starting with así emphasizes the result (“This way, dust won’t get in”) and then gives the instruction.

Why el polvo and not just polvo?

Spanish often uses the definite article with mass/generic nouns. El polvo here means “dust (in general).” You might see zero article (... no entra polvo) in some contexts, but with household talk about dust, el polvo is the usual, natural choice.

Is polvo masculine? Any pitfalls with plurals?

polvo is masculine singular: el polvo.
los polvos means “powders” (e.g., makeup: polvos). Also note the colloquial expression echar un polvo (sexual slang). Stick to el polvo for dust.

Can I change the word order in así no entra el polvo?

Así no entra el polvo (neutral, very natural).
Así el polvo no entra (also correct; slightly more contrastive).
El polvo no entra así changes the meaning to “Dust doesn’t enter like that,” which isn’t what you want here.

How do I make this more polite?

Informal: ¿Puedes/Podrías cerrar la puerta, por favor? Así no entra el polvo.
Formal: ¿Puede/Podría cerrar la puerta, por favor? Así no entra el polvo.
Adding por favor and using podrías/podría softens the request.

What about plural “you” in Spain?

Informal plural (vosotros): Cerrad la puerta, así no entra el polvo.
Formal/Latin-American plural (ustedes): Cierren la puerta, así no entra el polvo.
With a pronoun: Cerradla / Ciérrenla; negative: No la cerréis / No la cierren.

How do I pronounce this in Spain?

Cierra: initial “ci” as Castilian “th” + glide: roughly “THYE-rra,” with a trilled rr.
puerta: “PWEHR-ta.”
así: “ah-SEE” (stress on the second syllable).
entra: “EN-tra” (single tap r).
polvo: “POL-bo” (b and v sound the same).

Is there a colloquial alternative with que?

Yes: Cierra la puerta, que si no entra el polvo or Cierra la puerta, que entra el polvo. This que works like “because/otherwise” in everyday speech. It’s common and natural in Spain.