Los perros ladran cuando llega alguien a la casa.

In Spanish, when you talk about a group in general (dogs as a category), you usually use the definite article:

• Los perros ladran = Dogs (in general) bark.
• Los gatos duermen mucho = Cats sleep a lot.

Using Perros ladran is not standard in this kind of general statement; it sounds incomplete or like a fragment.

So Los perros here means “dogs in general”, not specific, identified dogs.

Ladrar is the infinitive (to bark).
Ladran is the present tense, 3rd person plural (they bark).

The subject is los perros (they), so the verb must agree:

• Yo ladro – I bark
• Tú ladras – You bark
• Él/Ella ladra – He/She barks
• Nosotros ladramos – We bark
• Vosotros ladráis – You (plural, Spain) bark
• Ellos/Ellas ladran – They bark

Here: Los perros ladran = The dogs bark / Dogs bark.

Spanish uses the present tense very often for habitual actions, where English might use:

• simple present: The dogs bark when…
• or will: The dogs will bark when…

Los perros ladran cuando… means “The dogs bark when(ever)…”, describing what typically happens.

Ladrarán (they will bark) would refer to a specific future situation, not a general habit:

• Los perros ladrarán cuando llegue el fontanero mañana.
  The dogs will bark when the plumber arrives tomorrow.

The Spanish word alguien is grammatically singular, just like someone in English. So the verb must also be singular:

• alguien llega = someone arrives
• alguien viene = someone comes

That’s why we have:

• Los perros ladran (plural, for los perros)
• cuando llega alguien (singular, for alguien)

Even though “someone” could be any person, it is one person at a time, so the grammar is singular.

Yes, you can say both:

• Los perros ladran cuando llega alguien a la casa.
• Los perros ladran cuando alguien llega a la casa.

Both are grammatically correct and mean the same thing.

The version llega alguien sounds a bit more natural or neutral in Spanish, and slightly emphasizes the arrival itself. The version alguien llega emphasizes “someone” a little more. The difference is subtle; in everyday speech, both are fine.

Both cuando + indicativo and cuando + subjuntivo exist, but they are used in different situations.

1. Habitual / general facts → Indicative

   • Los perros ladran cuando llega alguien a la casa.
     Whenever someone arrives, the dogs bark. It’s a repeated, known pattern.

2. Future / not-yet-real event → Subjunctive

   • Ladrarán cuando llegue alguien.
     They will bark when someone arrives (in the future, not yet happened).
   • Cuando llegue alguien, avísame.
     When someone arrives, let me know.

So in your sentence, it’s a habitual action, so the indicative (llega) is correct.

A and en express different ideas:

• a = movement toward a place (to)
• en = location in/at a place

Here, llega alguien a la casa describes someone arriving to the house (movement to that destination).

Compare:

• Llega alguien a la casa.
  Someone arrives at the house (comes to it).

• Hay alguien en la casa.
  There is someone in the house (already inside, location).

Both are correct, but they don’t always mean the same thing:

• a la casa = to the house (a specific house)
• a casa = (to) home

So:

• Los perros ladran cuando llega alguien a la casa.
  The dogs bark when someone arrives at the (particular) house.

• Los perros ladran cuando llego a casa.
  The dogs bark when I get home.

If you mean “our home” or “the place where we live,” a casa is very usual. If you are pointing out a particular house as a physical building (for example, in a story or description), a la casa fits well.

Because casa is a feminine noun:

• la casa = the house

The preposition a + la stays as a la.
The contraction al only happens with a + el (masculine singular):

• a + el coche → al coche (the car)
• a + el banco → al banco (the bank)

But for feminine:

• a + la casa → a la casa
• a + la playa → a la playa

Alguien is an indefinite pronoun meaning “someone / anyone”.

Important points:

• It is always singular. There is no plural “alguienes”.
• It does not change for gender: same form for male or female.
• For plural ideas, you use other expressions, such as:
  
  • algunas personas – some people
  • algunos / algunas – some (people), depending on context

Example:

• Los perros ladran cuando llega alguien a la casa.
  The dogs bark when someone arrives at the house.
• Los perros ladran cuando llegan algunas personas.
  The dogs bark when some people arrive.

The personal a is used when a person is the direct object, not when it is the subject.

• Subject:

  • Alguien llega a la casa.
    Someone arrives at the house. (alguien is the subject → no personal a)

• Direct object (then we use a):

  • Los perros ladran a alguien.
    The dogs bark at someone. (alguien is the direct object → personal a)

In your sentence, alguien is the subject of llega, so no personal a is needed.

In Spanish, you normally don’t put a comma before a cuando clause that gives essential information about time:

• Los perros ladran cuando llega alguien a la casa.
• Llamo a mi madre cuando salgo del trabajo.

A comma would usually be wrong here. You’d only use a comma if the cuando clause is clearly parenthetical or separated for special emphasis, which is less common and more stylistic. In normal writing, no comma is correct.

Yes, that word order is perfectly natural and very common:

• Los perros ladran cuando llega alguien a la casa.
• Cuando llega alguien a la casa, los perros ladran.

Both are correct and mean exactly the same. Spanish is quite flexible with placing the time clause (cuando…) either before or after the main clause. The version starting with Cuando… might feel a bit closer to English “When someone arrives at the house, the dogs bark.”