Mi perro es muy activo cuando corremos en el parque.

In Spanish, mi is a possessive adjective meaning my, used before nouns:

• mi perro = my dog
• mi casa = my house

Me is an object pronoun (me) and cannot go before a noun:

• Él me vio. = He saw me.

So you must say mi perro, never me perro.

Perro is masculine, perra is feminine. You choose based on the dog’s sex:

• Mi perro = my (male) dog
• Mi perra = my (female) dog

If you’re speaking generally and don’t care about the dog’s sex, many people default to perro, but if you know it’s a female, perra is more precise.

Spanish has two verbs for to be:

• ser (here: es) for more inherent or characteristic qualities
• estar (here: está) for more temporary states or conditions

In Mi perro es muy activo, being an active dog is presented as a characteristic of the dog, not just a temporary mood. So es is natural.

You could say Mi perro está muy activo if you wanted to emphasize his current level of activity (e.g. today he’s unusually active), but the original sentence sounds more like a general trait.

• muy = very and is used before adjectives and adverbs
  
  • muy activo, muy rápido, muy grande
• mucho = a lot / much / many, usually with verbs or nouns
  
  • corre mucho (he runs a lot)
  • tiene mucha energía (he has a lot of energy)

Because activo is an adjective, you must use muy activo, not mucho activo.

Adjectives must agree in gender and number with the noun they describe.

• perro is masculine singular → activo
• perra is feminine singular → activa

So:

• Mi perro es muy activo.
• Mi perra es muy activa.

You could say Él es muy activo, but you’d usually need more context so people know who él refers to.

• Mi perro es muy activo clearly tells you who is active.
• After that, you could continue with Él es muy activo cuando…, because now él is clear.

The original sentence just chooses to mention mi perro explicitly instead of a pronoun.

Corremos is the present tense, first person plural of correr (to run):

• nosotros corremos = we run

Here the present tense expresses a habitual action:
cuando corremos en el parque = when we run in the park (whenever we do it, in general, not right now only).

So the present is the natural tense for habits and routines in Spanish.

Spanish verb endings already show the subject:

• corro = I run
• corres = you run
• corre = he/she runs
• corremos = we run

Because corremos clearly indicates we, it’s normal to drop nosotros unless you want to emphasize it:

• Cuando corremos en el parque = When we run in the park
• Cuando nosotros corremos en el parque = When we (as opposed to others) run in the park

Both are grammatically correct; dropping nosotros is simply more common and natural.

Grammatically, corremos just means we run. Spanish doesn’t specify exactly who is included in we.

From context, we infer it’s the speaker + someone/something else. In everyday speech, this sentence is normally understood as:

• My dog is very active when we (my dog and I) run in the park.

If the speaker meant “I run while my dog runs around,” it would still be corremos in Spanish; the distinction is usually understood from context, not from grammar.

Both cuando and mientras can talk about time, but they differ slightly:

• cuando = when (at the time that)
  
  • Focus on the moment or situation: when we run
• mientras = while (during the time that)
  
  • Focus on simultaneity: at the same time as

In Mi perro es muy activo cuando corremos en el parque, the idea is:
He is very active at the times that we run in the park.
So cuando fits naturally.

You could say Mi perro está muy activo mientras corremos en el parque, focusing more on during the time we’re running, but that slightly changes the nuance and often would pair better with está (temporary action) rather than es (general trait).

With cuando, Spanish uses:

• Indicative for habitual actions or facts
  
  • Mi perro es muy activo cuando corremos en el parque.
    → Whenever we run there (habit/routine).
• Subjunctive for future or uncertain events
  
  • Mi perro va a estar muy activo cuando corramos en el parque.
    → When we run there (in the future), he will be very active.

In the given sentence, running in the park is a regular, real, repeated situation, so corremos (indicative) is the correct choice.

• en = in / at (location)
  
  • corremos en el parque = we run in the park
• a (often al = a + el) = to (movement toward a place)
  
  • Vamos al parque = we’re going to the park

The sentence talks about running inside the park, not going there, so en el parque is correct.

• el parque = the park
  
  • Usually implies a specific park both speaker and listener know (e.g., the usual park they go to).
• un parque = a park (any park, non-specific)

So:

• Mi perro es muy activo cuando corremos en el parque.
  → Probably refers to our usual park.
• Mi perro es muy activo cuando corremos en un parque.
  → In general, whenever we run in a park (any park), he’s very active.

Both are grammatically correct; the choice depends on whether you mean a specific park or any park.

Yes. Spanish allows that word order change without changing the basic meaning:

• Mi perro es muy activo cuando corremos en el parque.
• Cuando corremos en el parque, mi perro es muy activo.

Both are correct and natural. Starting with cuando… can emphasize the situation (the time/condition) a bit more.