Learn Hardcore Japanese: hutago ha itumo issyo ni kouen de asobu node, totemo naka ga ii desu. - The twins always play together in the park, so they get along very well.

双子 literally means “twins.” It usually refers to the pair as a unit, not to just one of the siblings. Japanese normally doesn’t mark plural the way English does, so 双子 can function like “the twins” here.

双子は uses は, the topic particle. It means “as for the twins,” setting them up as the thing we are talking about. The rest of the sentence tells us something about them: that they always play together and therefore get along well.

What does いつも mean, and where can it go in the sentence?

いつも means “always,” “usually,” or “all the time,” depending on context. In this sentence it modifies the action 遊ぶ (to play): they always play together.

The word order here is:

双子は いつも 一緒に公園で遊ぶ

You could move いつも a bit without changing the meaning much:

公園で いつも 一緒に遊ぶ
一緒に いつも 公園で遊ぶ (less common, but understandable)

It generally appears before the verb or right before the phrase closely tied to the verb. You wouldn’t normally put it after the verb (e.g., 遊ぶいつも is incorrect).

Why do we say 一緒に instead of just 一緒? What is the role of に here?

一緒 (いっしょ) by itself is a noun meaning “togetherness” or “being together.”
When you add に, it becomes an adverbial phrase 一緒に, meaning “together (with someone)” in the sense of doing something together.

In this sentence:

一緒に公園で遊ぶ
(to play together at the park)

一緒に modifies the verb 遊ぶ (“play”), telling us how they play: they play together.

Just 一緒 without に would usually need another structure, like:

一緒だ – “(They) are together.”
一緒の公園 – “the same park”

To describe doing an action together, you almost always use 一緒に + verb.

Why is the particle で used after 公園, and not に?

公園で uses で, which marks the place where an action takes place.

公園で遊ぶ = “to play at the park” (the park is the location of the activity)

The particle に with places more often marks direction, destination, or existence:

公園に行く – “to go to the park” (destination)
公園に子どもがいる – “There are children in/at the park” (existence)

Since 遊ぶ is an action done at a place, で is the natural choice: the playing happens in the setting of the park.

Why is it 遊ぶ (plain form) and not the polite form 遊びます, even though the sentence ends with です? Isn’t that mixing styles?

The clause before ので is:

公園で遊ぶので

In Japanese, when you use ので (“because / so”), the verb before ので is usually in the plain form (dictionary form, past, etc.), even in otherwise polite sentences. So:

公園で遊ぶので、…
公園で遊んだので、…

Then, the main clause at the end can be polite:

とても仲がいいです。

So the overall pattern is very common and natural:

[plain form clause] + ので、[polite clause]
公園で遊ぶので、とても仲がいいです。

This is not considered bad mixing; it’s standard usage. If you put 遊びます before ので, it sounds unnatural.

What does ので mean here, and how is it different from から?

In this sentence:

公園で遊ぶので、とても仲がいいです。

ので means “because” or “so.” Literally:

“Because they always play together at the park, they get along very well.”

Nuance compared to から:

ので often sounds more formal, softer, or more explanatory.
から can sound a bit more direct or casual, depending on context.

In many everyday cases, both can be used:

公園でいつも一緒に遊ぶから、とても仲がいいです。
公園でいつも一緒に遊ぶので、とても仲がいいです。

The meaning is almost the same, but ので here feels slightly more neutral/politer and less “punchy” than から.

What does 仲がいい literally mean, and what is 仲?

仲 (なか) is a noun meaning “relationship” or “relations” between people—how well they get along.

いい is the adjective “good.”

So 仲がいい literally means:

“(Their) relationship is good.”

In natural English, this is usually translated as:

“They get along well.”
“They are close.”
“They have a good relationship.”

So the end of the sentence:

とても仲がいいです。

literally: “Their relationship is very good.”
natural: “They get along very well.”

Why is it 仲がいい and not 仲はいい? What does the が do here?

仲がいい is a very common fixed pattern: [noun] がいい / 悪い / ある / ない, etc., where が marks the grammatical subject of the adjective.

仲がいい – “(Their) relationship is good.”
体が弱い – “(His) body is weak.”
運がいい – “(She) has good luck.”

You can say 仲はいい if you want to contrast it with something else:

ケンとジョンは、見た目は似ていないけど、仲はいい。
“Ken and John don’t look alike, but at least they get along.”

Here は highlights 仲 as a topic for contrast: “as for their relationship, it’s good.”

In your sentence, there is no contrast being made; it’s just a straightforward statement, so 仲がいい is the natural form.

What is the function of とても before 仲がいいです? Could we use something else there?

とても is an adverb meaning “very” or “really.” It modifies 仲がいい:

とても仲がいいです。
“They get along very well.”

You could replace とても with other degree adverbs:

すごく 仲がいい – “They get along really well” (more casual)
本当に 仲がいい – “They truly get along well”
まあまあ 仲がいい – “They get along so‑so” (different meaning, of course)

Leaving it out:

仲がいいです – “They get along well.” (no emphasis)

Can the order of いつも, 一緒に, and 公園で be changed? For example, is 双子は公園でいつも一緒に遊ぶので… okay?

Yes, Japanese allows some flexibility in the order of adverbial phrases. These are all natural:

双子は いつも一緒に公園で 遊ぶので…
双子は 公園でいつも一緒に 遊ぶので…
双子は いつも公園で一緒に 遊ぶので…

The nuance difference is very slight. Often, the earlier items are what the speaker wants to lightly foreground, but in everyday speech these variations are all acceptable.

What you generally avoid is splitting phrases in unnatural ways, like:

× 双子は一緒に公園でいつも遊ぶので… (not wrong grammatically, but feels awkward)

Keeping いつも, 一緒に, 公園で together before 遊ぶ is a good guideline.

Is there any hidden subject like “they” in the Japanese sentence, and how do we know who “they” is?

Japanese often omits pronouns when they’re clear from context. In this sentence, there is no explicit “they”:

双子はいつも一緒に公園で遊ぶので、とても仲がいいです。

The clause structure is:

Topic: 双子は – “as for the twins”
Verb: 遊ぶ – “(they) play”
Result: 仲がいいです – “(they) get along well”

The omitted subject for both 遊ぶ and 仲がいいです is simply understood as 双子 because that is the topic introduced with は. So you don’t need (and normally don’t use) a pronoun like 彼ら (“they”) here. The topic marking with は handles that function.