kouen de booru wo nageru to, inu ha sugu hirotte kimasu.

で marks the place where an action happens.

So in 公園で, it means in/at the park as the location where the ball-throwing happens.

Compare:

• 公園で遊ぶ = play in the park
• 公園に行く = go to the park

A common learner point is that で is for the place of an activity, while に often marks a destination or location of existence.

を marks the direct object of the verb.

In ボールを投げる, the ball is the thing being thrown, so it takes を.

• ボールを投げる = throw a ball
• 本を読む = read a book
• 水を飲む = drink water

So here, ボール is the object of 投げる.

It can feel like when or if, but the key point is that dictionary form + と often shows a natural, automatic, or expected result.

So:

• ボールを投げると、犬はすぐ拾って来ます。

has the sense of:

• When/if you throw the ball, the dog immediately picks it up and brings it back.

This と is often used for things that regularly happen or follow as a matter of course.

Examples:

• 春になると、暖かくなる。
  When spring comes, it gets warm.
• このボタンを押すと、ドアが開きます。
  If/when you press this button, the door opens.

So here it sounds like a habitual consequence, not just a random possibility.

は marks 犬 as the topic: as for the dog, this is what it does.

So 犬はすぐ拾って来ます means something like:

• As for the dog, it immediately picks it up and brings it back.

Using は makes the sentence sound more like a general description or habitual statement.

If you used 犬が, it would focus more directly on the dog as the subject, possibly as new or emphasized information.

Very roughly:

• 犬は = speaking about the dog
• 犬が = the dog is the one who...

In this sentence, は fits well because the speaker is describing what the dog typically does.

すぐ means immediately, right away, or soon.

In this sentence, it modifies the action that follows:

• 犬はすぐ拾って来ます。
  The dog immediately picks it up and brings it back.

Japanese adverbs like すぐ are often placed before the verb phrase they modify. Its position here is very natural.

This is the te-form + 来る pattern:

• 拾う = to pick up
• 拾って = te-form of 拾う
• 来ます = comes

Together, 拾って来ます means:

• pick it up and come
• more naturally in English: pick it up and bring it back

The idea is that the dog first picks up the ball, then comes back toward the speaker with it.

This pattern is very common:

• 持って来る = bring
• 買って来る = go buy and bring back / buy and come back
• 取って来る = fetch and bring back

So 拾って来ます is very natural for a dog fetching a ball.

Because the movement is understood as being toward the speaker or the speaker’s point of reference.

• 来る = come
• 行く = go

The dog goes to the ball, picks it up, and then returns to where the speaker is, so Japanese uses 来ます.

That is why English often translates this as bring it back rather than just go with it.

Because Japanese often omits information that is already clear from context.

The ball was already introduced in:

• ボールを投げる

So in the second part, it is understood that the dog picks up the ball. Japanese does not need to repeat it.

A fuller version could be:

• 公園でボールを投げると、犬はすぐボールを拾って来ます。

But this sounds more repetitive. Omitting the repeated object is more natural.

Japanese -ます forms do not map exactly to English tense labels. 来ます can mean present or future depending on context.

In this sentence, the whole statement sounds habitual/generic:

• Whenever the ball is thrown in the park, the dog immediately fetches it.

So 来ます is best understood here as describing something that regularly happens.

If you wanted to describe one specific past event, you would use past forms:

• 公園でボールを投げると、犬はすぐ拾って来ました。
  When I threw the ball in the park, the dog immediately picked it up and brought it back.

The subject is simply omitted, which is very common in Japanese.

In this sentence, the thrower could be understood as:

• I
• you
• someone
• or just a general person in that situation

Japanese often leaves out the subject when it is obvious, unimportant, or meant to stay general.

So the sentence is not focused on who throws the ball. The main point is the result: the dog fetches it right away.

It most naturally describes a usual pattern or repeated behavior.

That feeling comes mainly from:

• dictionary form + と, which often expresses a regular result
• 来ます, which here sounds non-past/habitual

So the sentence feels like:

• When you throw a ball in the park, the dog always fetches it right away.

If you wanted to strongly emphasize a single event, Japanese would usually use a different structure or more context.