kouen de nigeta koinu wo tukamaeruno wo tetudaimasita

A useful way to break it up is:

• 公園で = in the park
• 逃げた子犬 = the puppy that ran away / escaped
• 子犬を捕まえる = catch the puppy
• 子犬を捕まえるの = the act of catching the puppy
• 子犬を捕まえるのを手伝いました = helped with catching the puppy / helped catch the puppy

So the overall structure is basically:

[in the park] [the puppy that ran away] [catching it] [helped]

A smooth English rendering is something like I helped catch the puppy that ran away in the park.

で marks the place where an action happens.

So 公園で means in the park as the location of the action.

Here, it most naturally connects to 逃げた, giving:

• 公園で逃げた子犬 = the puppy that ran away in the park

If you used に, it would suggest a different kind of relationship, such as destination or existence, not the location where the action of running away happened.

In Japanese, descriptions come before the noun they describe.

So instead of saying:

• the puppy that ran away

Japanese says:

• ran-away puppy
• 逃げた子犬

This is called a relative clause. Japanese does not need a word like that or which here.

So:

• 逃げた子犬 = the puppy that ran away

It can correspond to either of those in English, depending on how natural you want the translation to sound.

Grammatically, 逃げた子犬 means:

• the puppy that escaped
• the puppy that ran away

In natural English, people might also say:

• the runaway puppy

But the Japanese structure is simply a verb phrase modifying a noun.

Because the escaping happened before the helping/catching.

The main verb is 手伝いました = helped.
Before that happened, the puppy had already run away, so Japanese uses the past form 逃げた.

So the logic is:

1. the puppy ran away
2. someone helped catch it

That is why 逃げた子犬 makes sense here.

Because here 捕まえる is being turned into a noun-like phrase by の.

• 子犬を捕まえる = to catch the puppy
• 子犬を捕まえるの = catching the puppy / the act of catching the puppy

The dictionary form often appears before の or こと when talking about an action as a thing.

So this is not saying caught the puppy as a completed main action. It is referring to the action of catching the puppy, which is what was helped.

It is nominalizing the verb phrase.

That means it turns:

• 子犬を捕まえる = catch the puppy

into something noun-like:

• 子犬を捕まえるの = catching the puppy / the act of catching the puppy

This is necessary because 手伝う often takes an action as the thing being helped:

• 〜のを手伝う = help with doing ~

So the sentence literally has the sense of:

• helped the catching of the puppy

which becomes natural English helped catch the puppy.

Because there are two different objects at two different levels.

First を

• 子犬を捕まえる
• を marks 子犬 as the direct object of 捕まえる
• so: catch the puppy

Second を

• 子犬を捕まえるのを手伝いました
• this を marks the whole phrase 子犬を捕まえるの as the object of 手伝いました
• so: helped with catching the puppy

So the structure is:

• [puppy] を [catch]
• [catching the puppy] を [helped]

That is why you see を twice.

手伝いました is the polite past form of 手伝う, which means to help or to assist.

In this pattern:

• V-る のを 手伝う

it means:

• help do V
• help with doing V

So:

• 子犬を捕まえるのを手伝いました
• = helped catch the puppy

The politeness comes from -ました, and the past meaning comes from the same ending.

The subject is not stated.

Japanese very often leaves out the subject when it is understood from context. So this could mean:

• I helped catch the puppy
• We helped catch the puppy
• He/She helped catch the puppy

depending on the situation.

The sentence only clearly tells you that someone helped.

No, that is also omitted.

English often wants something like:

• I helped them catch the puppy

But Japanese does not have to say them if it is obvious from context.

So 子犬を捕まえるのを手伝いました just says that someone helped with the action of catching the puppy. It does not explicitly say who the main catcher was.

Yes, 子犬を捕まえることを手伝いました is grammatically possible.

However, の is more natural here because it refers to a more concrete, immediate action. こと can sound a bit more abstract or formal.

So in this sentence:

• 捕まえるのを手伝いました sounds more natural than
• 捕まえることを手伝いました

No. In normal Japanese writing, it would usually be written as:

公園で逃げた子犬を捕まえるのを手伝いました

The spaces are typically added only for learners to make the parts easier to see.

In theory, context can sometimes create ambiguity, but in this sentence the most natural reading is:

• 公園で逃げた子犬
• the puppy that ran away in the park

That is because 公園で is directly attached to the relative clause 逃げた.

If you wanted to make it clearer that the catching happened in the park, you would usually phrase it differently. As written, most learners should read 公園で as modifying 逃げた.