La guardia abrió la puerta.

Guardia is a noun that can refer to a person, and the article tells you the person's sex:

• el guardia = the male guard
• la guardia = the female guard

So in this sentence, la guardia most naturally means the female guard.

A useful detail: the noun itself stays guardia either way. Only the article changes.

Not always in every context, but in a simple sentence like this, that is the most likely reading.

Depending on context, guardia can also refer to:

• a guard or police officer
• guard duty
• a guard/watch as a shift or period of duty

But here, because it is followed by a normal action verb (abrió) and the sentence looks like a person doing something, the natural reading is the female guard opened the door.

Abrió is the preterite (also called the simple past) of abrir.

It is:

• third person singular
• meaning he opened, she opened, or it opened

In this sentence, it matches la guardia, so it means she opened.

The preterite is used here because the action is seen as completed: the guard opened the door.

Because abrió and abría do not mean exactly the same thing.

• abrió = opened / a completed action
• abría = was opening or used to open / background, ongoing, or habitual action

So:

• La guardia abrió la puerta. = The guard opened the door.
• La guardia abría la puerta. = The guard was opening the door / used to open the door.

In a short standalone sentence describing one finished event, abrió is the normal choice.

Yes. In the preterite, abrir is regular:

• yo abrí
• tú abriste
• él/ella/usted abrió
• nosotros abrimos
• vosotros abristeis
• ellos/ellas/ustedes abrieron

So abrió is exactly what you would expect from a regular -ir verb.

One thing to watch out for: the past participle is irregular:

• abrir → abierto

So:

• abrió = simple past, regular
• ha abierto = present perfect, irregular participle

The accent mark shows the correct stress:

• abrió is stressed on the last syllable: roughly ah-BRYOH

Without the accent, the spelling would not be correct for this verb form.

This is normal in the preterite:

• habló
• comió
• abrió

So the accent is not optional; it is part of the correct spelling.

Because la puerta means the door, not a door.

• la puerta = a specific door, usually known from the context
• una puerta = any door / a door that is not being identified as specific

So:

• La guardia abrió la puerta. = The guard opened the door.
• La guardia abrió una puerta. = The guard opened a door.

Because puerta is simply a feminine noun in Spanish.

That is why it takes:

• la puerta
• not el puerta

Like all nouns, it has grammatical gender, and the article has to agree with it.

This is the most neutral and standard word order:

• La guardia = subject
• abrió = verb
• la puerta = direct object

So it follows the common subject + verb + object pattern.

Spanish can change the order for emphasis, but that sounds more marked, for example:

• Abrió la puerta la guardia.

That is possible, but much less neutral than La guardia abrió la puerta.

Yes. Spanish often drops the subject when it is clear from the context.

So Abrió la puerta can mean:

• He opened the door
• She opened the door
• You opened the door (formal usted)

Adding la guardia makes the subject explicit and removes the ambiguity.