Learn Hardcore Japanese: inu ha kouen de genki ni hasirimasu. - The dog runs energetically in the park.

犬は sets up the dog / dogs as the topic: As for the dog(s)…
The rest of the sentence, 公園で元気に走ります, is what you want to say about that topic.

は is often used when:

You are making a general statement (for example, about dogs in general).
The thing is already known in the conversation, or you are introducing a broad topic.

If you said 犬が公園で元気に走ります, with が:

が tends to mark a specific subject and has a more focus / new information feel.
It can sound more like: It is the dog that runs energetically in the park, possibly contrasting with something else (e.g., the cat does not).

In many simple sentences either は or が is grammatically possible, but they give slightly different nuances. Here, 犬は feels like a neutral topic: As for the dog(s)…

Why is it 公園で and not 公園に?

Both で and に can appear after a place, but they have different roles.

で marks the location where an action takes place.
- 公園で走ります = run *in the park* (the running happens in that location).
に often marks:
- A destination / goal:
  - 公園に行きます = go to the park
- A point in time:
  - 三時に行きます = go at three o’clock

You usually say:

公園で走ります (run in the park – action location)
but:
公園に走ります would sound like run *toward the park* (treating the park as the destination), which is a different meaning.

So で is correct here because the running is taking place inside the park.

What exactly is 元気に doing in this sentence? Is 元気 an adjective or a noun?

元気 is originally a な-adjective / noun-like word meaning healthy, lively, full of energy.

As a basic form:
- 元気です = is healthy / energetic
- 元気な犬 = a lively dog

When you add に to many な-adjectives (and to some nouns), they become adverb-like, modifying the verb:

元気に走ります
- Literally: run in a healthy/lively way
- Natural English: run energetically / run actively

So grammatically:

元気: a な-adjective (and also used as a noun).
元気に: an adverbial form modifying 走ります.

Why is the verb 走ります instead of 走る?

走ります is the polite present/future form (ます-form) of the verb 走る (to run).

走る – dictionary/plain form
走ります – polite form used in:
- talking to strangers
- talking to teachers, bosses, customers
- textbooks and formal examples

Basic pattern for a る-ending dictionary form like 走る:

Dictionary: 走る
Polite: 走ります
Past polite: 走りました
Negative polite: 走りません

In casual speech with friends, you might say:

犬は公園で元気に走る。 (plain form)

The meaning is the same; the difference is politeness level.

Can I change the word order, or does it have to be 犬は公園で元気に走ります?

Japanese word order is more flexible than English because particles show each word’s role. The verb usually comes at the end, but elements before it can often be rearranged.

All of these are possible and natural, with small nuance differences:

犬は公園で元気に走ります。
犬は元気に公園で走ります。
公園で犬は元気に走ります。
公園で元気に犬は走ります。 (less common but still understandable)

However:

Putting the verb anywhere except the end is usually wrong in normal sentences.
The most neutral and natural here is the original:
- 犬は公園で元気に走ります。

So you have some flexibility before the verb, but the verb goes last in standard sentences.

Is 犬 singular or plural here? Does it mean a dog or dogs?

Japanese does not normally mark singular vs plural the way English does.

犬 can mean:
- a dog
- the dog
- dogs
- the dogs

The exact meaning depends on context:

If you are talking about your pet:
- It probably means my dog.
If you are speaking generally about the species:
- It means dogs in general.

You can make it explicitly plural with 犬たち, but that is usually for specific identifiable dogs, not a general statement.
So 犬は公園で元気に走ります could be translated as:

Dogs run energetically in the park.
or
The dog runs energetically in the park.

depending on what has already been mentioned in the conversation.

Why is the particle は written as は but pronounced わ?

The syllable は is normally pronounced ha, but when it is used as the topic particle, it is pronounced wa.

As part of a word:
- はな (flower) → pronounced hana
As the topic marker:
- 犬は → pronounced いぬわ (inu wa)

This is due to historical spelling conventions:

A long time ago, the pronunciation and spelling matched more closely.
Modern pronunciation changed, but the traditional spelling was kept for some particles.

Other examples:

へ as a particle (direction marker) is written へ but pronounced え (e).
を (object marker) is now pronounced お (o), but written with a special kana.

So: は = ha in words, wa when it is the topic particle.

What is the difference between 元気に走ります and something like 速く走ります?

Both phrases modify 走ります, but they focus on different aspects:

元気に走ります
- Emphasis: mood / energy / liveliness
- Means: runs energetically, cheerfully, actively
- Suggests the dog is full of life and happy.
速く走ります (from 速い = fast)
- Emphasis: speed
- Means: runs fast / runs quickly
- Says nothing about the dog’s mood, just its speed.

So:

Use 元気に走ります when you want to describe how lively the running looks.
Use 速く走ります when you want to describe how fast the running is.

How would I say this if I want to emphasize that the dog is running right now, not just in general?

To emphasize an action that is in progress right now, Japanese often uses the 〜ている form.

Starting from 走ります:

て-form of 走る: 走って
Progressive polite: 走っています

So:

犬は公園で元気に走っています。
- Emphasis: The dog is running energetically in the park (right now).

Compare:

犬は公園で元気に走ります。
- More like a habitual / general statement:
  - The dog (tends to) run energetically in the park.
  - Dogs run energetically in the park.

Could any words be left out in natural conversation?

Yes. Japanese often omits things that are clear from context.

For example, if both speakers already know you are talking about your dog:

You could say just:
- 公園で元気に走ります。
  - [It] runs energetically in the park.

Or, if the location is obvious (you are literally in the park while speaking), you might drop 公園で:

犬は元気に走ります。
- The dog runs energetically.

The topic (犬は) is very commonly dropped after it has been mentioned once and is understood. Particles inside the sentence (like で and に) are usually not dropped, because they show the grammatical role of words and are important for clarity.