Learn Hardcore Japanese: syougakusei no koro, mainiti kouen de tomodati to issyo ni asobimasita. - When I was an elementary school student, I played in the park with my friends every day.

Literally, 小学生のころ is the time of [being] an elementary school student, which we naturally translate as when I was in elementary school or when I was an elementary school student.

Same pattern appears in:

子どものころ – when I was a child
学生のころ – when I was a student

What exactly does ころ mean, and how is it different from とき?

ころ means around that time / that period (of life). It often suggests a somewhat broad or vague span, especially ages or life stages.

小学生のころ = during the period when I was an elementary school student (the whole era, not one specific day).

Compared with とき:

小学生のとき – when I was an elementary school student (more neutral)
小学生のころ – when I was an elementary school kid (often feels like a whole era, a bit softer/vaguer)

In many cases, ころ and とき are interchangeable here, and 小学生のころ、毎日… and 小学生のとき、毎日… are both natural.

Where is the subject I in this sentence?

It is omitted. Japanese often leaves out the subject (and even the object) when it is clear from context.

The full version could be:

（私は）小学生のころ、毎日公園で友達と一緒に遊びました。

But 私は is usually dropped if it is obvious we are talking about the speaker. Mentioning 私 every time feels repetitive and unnatural in Japanese.

Why is there no particle after 毎日?

Time expressions like 毎日 (every day), 昨日 (yesterday), 来年 (next year) often appear without a particle.

So:

毎日公園で友達と一緒に遊びました。 – perfectly normal

You can add particles, but they add nuance:

毎日は公園で友達と一緒に遊びました。
– emphasizes every day as a contrast (e.g., every day I played, but on Sundays I did something else)
毎日に公園で…
– generally unnatural here; に with time is common for specific points (e.g., ３時に), not a vague frequency word like 毎日.

Why is で used after 公園 instead of に?

で marks the place where an action happens.

公園で遊びました。
– I played at the park. (the park is the location of the action)

By contrast, に often marks:

destination / goal of movement:
- 公園に行きました。 – I went to the park.
location of existence:
- 公園に犬がいます。 – There is a dog in the park.

So with 遊ぶ (to play / hang out), you use で because you are doing the action at that place.

What does the particle と after 友達 mean here?

Here と is the companion marker: with.

友達と遊びました。 – I played with my friends.

This と can also mean and (AとB), or mark quotes, or form certain grammar patterns, but in this sentence it clearly means with (together with someone).

You cannot replace it with に here; 友達に遊びました is ungrammatical.

Why is there に after 一緒? Is 一緒に one word?

一緒に functions as a fixed adverbial phrase meaning together.

Structurally:

一緒 – together, same
に – adverb‑forming particle here

So 一緒に遊びました = played together.

In practice, learners can treat 一緒に as if it were one chunk:

友達と一緒に行きました。 – I went together with my friend.
家族と一緒に住んでいます。 – I live together with my family.

Using 一緒 without に before a verb is not natural in this meaning.

Why is 遊びました used instead of something like 遊んでいました for a repeated past action?

Japanese past polite (遊びました) covers both:

a single past event
a repeated or habitual past action

Context and adverbs supply the nuance. Here 毎日 (every day) signals that this was habitual, so 遊びました is understood like I used to play.

遊んでいました (past progressive / past continuous) would emphasize ongoing state in the past, or background action:

小学生のころ、毎日公園で友達と一緒に遊んでいました。
– Also natural; it paints the picture as a continuous habit a bit more vividly, but the basic meaning is very similar.

Can the word order change? For example, where else can 毎日 go?

Yes, Japanese word order is quite flexible as long as particles stay with the right words. Some natural variations:

小学生のころ、毎日公園で友達と一緒に遊びました。 (original)
小学生のころ、公園で毎日友達と一緒に遊びました。
毎日、小学生のころは公園で友達と一緒に遊びました。

Differences are subtle and mostly about emphasis or rhythm. The original is very typical and easy to parse. Just keep in mind: don’t separate a noun from its particle (e.g., 公園で, 友達と).

What level of politeness is 遊びました, and how would you say this casually?

遊びました is the polite past form of 遊ぶ. It is appropriate for talking to people you are not very close to, in writing, or in formal situations.

Casual past is 遊んだ. A fully casual version would be:

小学生のころ、毎日公園で友達と一緒に遊んだ。

You would say this to close friends or family, or in informal writing like a diary.

Does 遊ぶ mean playing kids’ games only, or can it also mean just hanging out?

遊ぶ has a wide range:

Children’s play
- 子どもが外で遊んでいます。 – The kids are playing outside.
Hanging out, having fun, going out (even for adults)
- 友達と遊ぶ – hang out with friends
- 週末は東京で遊びました。 – I went out / had fun in Tokyo on the weekend.

In your sentence, 友達と一緒に遊びました likely means both played (as a kid) and generally spent fun time with friends.

How could I say the same kind of sentence about a different school level, like high school?

You can reuse the same pattern [school level] のころ. For example:

高校生のころ、毎日公園で友達と一緒に遊びました。
– When I was in high school, I played / hung out with my friends in the park every day.

Other common variants:

中学生のころ – when I was a junior high / middle school student
大学生のころ – when I was a university student