kouen de kodomo ga booru wo nageru to, inu ha nando mo oikakemasu.

で marks the place where an action happens. In this sentence, 公園で means in the park as the location of the throwing.

A useful comparison:

• 公園で = in the park / at the park, where someone does something
• 公園に = to the park / at the park, often for destination or existence

So here で is used because 投げる is an action taking place there.

が marks 子供 as the subject of the first action, 投げる.

In this sentence, the structure is:

• 子供が = the child / children are the ones who throw
• 犬は = as for the dog, it repeatedly chases

Using が in the first clause helps identify who performs that action. Using は for 犬 makes the dog the main topic of the second clause.

So the sentence is roughly structured like:

• When children throw a ball in the park, as for the dog, it chases it repeatedly.

は marks the topic. Here, after the first clause sets up the situation, 犬は shifts attention to what the dog does in that situation.

This gives a nuance like:

• As for the dog, it chases it many times.

If you said 犬が, it would sound more like you are specifically identifying the dog as the subject, rather than presenting it as the topic. Both can be possible in different contexts, but 犬は is very natural here.

The pattern dictionary form + と often means when or whenever.

So ボールを投げると means:

• when someone throws a ball
• or whenever someone throws a ball

This と is often used for things that happen naturally, repeatedly, or as a regular result. In this sentence, it suggests a typical reaction: when the ball is thrown, the dog chases it.

Before と, Japanese normally uses the plain form, not the polite ます form.

So:

• 投げると = correct
• 投げますと = not natural in ordinary grammar for this pattern

The sentence still ends politely with 追いかけます, because the final verb sets the politeness level of the whole sentence. This is very common in Japanese: plain form inside the sentence, polite form at the end.

Not necessarily. Japanese non-past can express:

• present
• future
• habitual action
• general truth

Here it sounds habitual or repeated because of と and 何度も. The meaning is not just one future event, but more like:

• Whenever the children throw a ball in the park, the dog chases it repeatedly.

So 追いかけます here describes a usual reaction.

何度も means many times, repeatedly, or again and again.

It emphasizes repetition. In this sentence, it tells us the dog does not just chase once; it keeps doing it over and over.

A close English sense would be:

• repeatedly
• over and over
• many times

The object is omitted because it is understood from context. The most natural understood object is ボール.

So even though the sentence literally says:

• the dog chases repeatedly

Japanese often leaves out words that are obvious. A fuller version could be:

• 犬はボールを何度も追いかけます。

But since ボール was already mentioned, leaving it out sounds natural.

It can be either. Japanese nouns usually do not have to show singular or plural.

So 子供 can mean:

• a child
• children

You understand the number from context. Since the English meaning has already been given, you know which one is intended there, but grammatically the Japanese form itself does not force singular or plural.

何度も is an adverb, and adverbs in Japanese are often placed before the verb they modify. So:

• 犬は何度も追いかけます = very natural

You may also hear similar adverbs in slightly different positions depending on emphasis, but placing 何度も before 追いかけます is the standard and easiest pattern to learn.

Yes, there is a slight difference.

• 追う = to chase, pursue
• 追いかける = to go after, chase after

In many contexts they are similar, but 追いかける often feels more vivid and physical, as if the dog is actively running after the ball. It is a very natural verb choice for this kind of situation.

The comma is just punctuation to make the sentence easier to read. It separates the when/whenever clause from the main clause:

• 公園で子供がボールを投げると、
• 犬は何度も追いかけます。

It does not change the grammar. Japanese punctuation is often lighter and more flexible than English punctuation, but this comma is helpful and natural here.