Learn Hardcore Spanish (Latin America): Para reforzar la puerta, el dueño usa un candado. - To reinforce the door, the owner uses a padlock.

Because para + infinitive expresses purpose: “in order to.” Para reforzar la puerta = “In order to reinforce the door.” Using por + infinitive would express cause (“because of”) and would be odd here: Por reforzar la puerta… ≈ “Because of reinforcing the door…,” which isn’t the intended meaning.

Is the comma after Para reforzar la puerta required?

It’s optional. A comma is common when a purpose clause comes first: Para reforzar la puerta, el dueño usa un candado. Without the comma is also acceptable. If the purpose goes at the end, you normally don’t use a comma: El dueño usa un candado para reforzar la puerta.

Can I put the purpose clause at the end instead?

Yes. El dueño usa un candado para reforzar la puerta is perfectly natural and probably more common in everyday speech.

What’s the difference between para + infinitive and para que + subjunctive?

Use para + infinitive when the subject is the same: El dueño usa un candado para reforzar la puerta.
Use para que + subjunctive when the subject changes or to express a desired outcome: El dueño usa un candado para que nadie entre.

Is reforzar the best verb if I just mean “to secure/lock” the door?

Reforzar suggests strengthening or adding extra protection. If you simply mean “to secure/lock,” more idiomatic options are:

asegurar la puerta
cerrar con llave
asegurar la puerta con un candado / ponerle un candado a la puerta

The original sentence is fine if you mean “reinforce the door’s security.”

Does reforzar have any irregularities?

Yes:

Present tense stem change (o → ue): yo refuerzo, tú refuerzas, él refuerza, nosotros reforzamos, ellos refuerzan.
Spelling change in the first-person preterite: yo reforcé (z → c before e). Everything else is regular.

Why is it la puerta and not a la puerta?

Because reforzar takes a direct object, and inanimate direct objects don’t use the personal a: reforzar la puerta. You’ll see a la puerta with verbs that take an indirect object or certain set phrases, e.g., ponerle un candado a la puerta, llamar a la puerta.

Does dueño mean “owner” or “landlord”? What about feminine?

Dueño generally means “owner.” For “landlord,” you’ll often hear propietario, arrendador, or regionally casero. Feminine forms are dueña, propietaria, arrendadora, casera. Context determines which is best.

Why does el in el dueño have no accent? Isn’t it él?

El (no accent) is the definite article “the.” Él (with accent) is the pronoun “he.” Here we need the article: el dueño = “the owner.” Example contrast: Él usa un candado = “He uses a padlock.”

Is usa the best verb, or is there something more idiomatic?

Usa is fine and neutral. Very idiomatic alternatives are:

poner un candado / ponerle un candado a la puerta
asegurar la puerta con un candado Regional options include echar candado or cerrar con candado. For built-in locks (not padlocks), cerrar con llave is common.

What’s the difference between candado, cerradura, cerrojo, and pestillo/pasador? And what about chapa?

Candado: padlock (portable lock with a shackle).
Cerradura: the built-in lock mechanism in the door (what you use a key in).
Cerrojo: bolt/deadbolt that slides into place (may be keyed or not).
Pestillo/Pasador: latch or simple slide bolt (usually inside).
Chapa: in parts of Latin America (e.g., Mexico), this can mean the lock/door hardware; usage varies by region.

Why un candado and not el candado?

Un candado introduces a new, non-specific padlock (“a padlock”). El candado would refer to a specific one already known in context (“the padlock”).

Can I omit the article and say usa candado?

Generally no; you need un with a singular countable noun: usa un candado. You can omit the article in set phrases or with some uncountable/plural uses, but not here. You can say sin candado (“without a padlock”) because that’s a different structure.

How can I avoid repeating la puerta with pronouns?

Two common ways, depending on the verb:

With reforzar (direct object): Para reforzarla, el dueño usa un candado. (la replaces la puerta.)
With poner … a la puerta (indirect object): El dueño le pone un candado (a la puerta). (le replaces a la puerta.) Note that after an infinitive, the pronoun attaches: reforzarla.

How do you pronounce dueño, and what is the ñ?

Ñ/ñ is its own letter, pronounced like the “ny” in “canyon.” Dueño sounds like “DWEH-nyoh.” In Latin America, z in reforzar sounds like an “s” (reh-fohr-SAR).

Is fortalecer a good synonym for reforzar here?

Not usually for a door. Fortalecer tends to be used for abstract things (relationships, economy) or large structures. For a door, reforzar la puerta or asegurar la puerta are the natural choices.