Learn Hardcore Japanese: koukou no toki, watasi ha hutago no imouto to tama ni kenka wo simasita. - When I was in high school, I sometimes fought with my twin sister.

But とき itself is not the action; it’s a noun meaning “time”, so we don’t use で between 高校 and とき.
(You can say 高校のときに with に, but that’s a different question—see below.)

Does 高校のとき mean “in high school” or “when I was a high school student”? How is it different from 高校生のとき?

Both are very close in meaning, but there is a nuance:

高校のとき
Literally “the time of high school.”
→ Feels like “back in high school / during my high-school years.”
It can refer to that period of life in a broad way.
高校生のとき
Literally “the time (when I was) a high school student.”
→ Slightly more focused on your status as a student.

In everyday conversation, people often use them almost interchangeably.
If you want to be very explicit that you were a student (and not, say, a teacher at a high school), 高校生のとき makes that extra clear.

Can I say 高校のときに instead of 高校のとき? What does the に add?

Yes, 高校のときに、私は双子の妹とたまにけんかをしました。 is perfectly natural.

とき is a noun meaning “time,” and に can mark a point or period in time.
So 高校のときに emphasizes “at that time (when I was in high school)” as the time when the action happened.

Difference in feel:

高校のとき、… – neutral, very common.
高校のときに、… – slightly more explicit that the action happened at that time.

In many cases the difference is very small, and both are acceptable.

Why is 私 marked with は here? Could I drop 私 or use が instead?

は marks the topic: what the sentence is “about.”
私 is obvious from context in many conversations, so:
- (私は) 双子の妹とたまにけんかをしました。
  Omitting 私 is completely natural in Japanese.

Using は:

私は双子の妹とたまにけんかをしました。
→ “As for me, I sometimes fought with my twin younger sister.”
It just sets “I” as the topic; no special contrast is required, but it allows contrast if needed (e.g., “I did, but my brother didn’t”).

Using が:

私が双子の妹とたまにけんかをしました。
This would sound like you are emphasizing “it was I (not someone else)” who fought, or answering “Who was it that fought with her?”
In the original sentence, that kind of emphasis is not needed, so は (or dropping 私) is more natural.

In 双子の妹, what does の mean, and what exactly does this phrase mean?

Here の links 双子 (“twin / twins”) to 妹 (“younger sister”), making 双子 an attribute of 妹:

双子の妹 = “younger sister who is a twin” → “my twin younger sister”

So the structure is:

[Type/description] の [family term]

Other similar expressions:

双子の兄 – twin older brother
双子の姉 – twin older sister
双子の弟 – twin younger brother

The の is like a possessive/descriptor link; it’s not “of” in a strict ownership sense, but rather “the [妹] that is a [双子].”

How do we know it’s a younger sister? With twins we’re the same age—why use 妹?

In Japanese kinship terms, 妹 always means “younger sister” (younger than the speaker).
Even with twins, there is still a birth order—one was technically born first.

If you are the older twin and your twin is a girl:
- You call her 妹 → 双子の妹.
If you are the younger twin and your twin is a girl:
- You call her 姉 (older sister) → 双子の姉.

So 双子の妹 implies:

The speaker is older than the twin sister (by birth order), even if the actual age difference is just a few minutes.

What is the particle と doing after 妹? Is this the same と as “and”?

Yes, it’s the same particle と, but here it’s used in its “with” (comitative) function.

妹とけんかをしました。
= “I had a fight with my younger sister.”

So X と V する can mean “do V with X”:

友だちと遊ぶ – play with a friend
先生と話す – talk with the teacher
双子の妹とけんかする – fight with my twin younger sister

This is different from listing nouns:

りんごとバナナ – apples and bananas

Same particle, different usage (listing vs. partner/comrade in an action).

What does たまに mean exactly? How is it different from ときどき or よく?

All three express frequency, but with different strengths:

たまに – “once in a while,” “occasionally”
→ Low frequency. Suggests it does happen, but not very often.
ときどき（時々） – “sometimes”
→ Neutral “sometimes,” more frequent than たまに.
よく – “often,” “frequently”
→ High frequency.

So in this sentence:

たまにけんかをしました。
= “We fought once in a while / occasionally,”
implying that fighting was not the usual state, just an occasional event.

Why is there a に in たまに? Can I just say たま?

In modern standard Japanese, the natural adverb is たまに.

Historically, たま is related to the idea of “rare” or “occasional,” and に turns it into an adverb:
たま（rare） + に → たまに (rarely / occasionally)

In everyday speech:

たまに – correct, standard adverb: “once in a while.”
たま by itself as an adverb sounds either:
- dialectal, or
- very casual/slangy in some contexts.

So for learners, just remember たまに as a fixed adverb meaning “occasionally.”

Why is there an を in けんかをしました? I’ve also seen けんかしました. Which is correct?

Both are correct:

けんかをする
けんかする

けんか is originally a noun (“a fight, quarrel”) that combines with する (“to do”) to form a verb.

けんか + を + する – “to do a fight” → “to fight”
Over time, this became a single verb: けんかする, and the を is often dropped.

This happens with many noun + する verbs:

勉強(を)する – to study
運動(を)する – to exercise
練習(を)する – to practice

Nuance:

けんかをしました can sound a bit more formal/explicit about it being “a fight (as a noun) that we did.”
けんかしました is very common and completely natural in conversation.

In this sentence, either form is fine.

Is けんかする transitive? Do I ever say 妹をけんかする?

You should not say 妹をけんかする. That is ungrammatical.

The usual pattern is:

A とけんかする – “A and (I) fight” / “fight with A”

So in your sentence:

双子の妹とけんかをしました。
= “I had a fight with my twin younger sister.”

You can omit the partner if it’s obvious:

よくけんかしました。 – We fought a lot. (Partner understood from context.)

But if you mention the partner, use と, not を.

Can I move たまに to other places in the sentence? For example, たまに双子の妹とけんかをしました—is that okay?

Yes, たまに is quite flexible in position. These are all natural:

高校のとき、たまに双子の妹とけんかをしました。
高校のとき、双子の妹とたまにけんかをしました。
たまに、高校のとき双子の妹とけんかをしました。 (a bit less common, but possible in context)

The main points:

たまに usually comes:
- at the start of the sentence, or
- just before the main verb phrase (here, before けんかをしました or near it).
Don’t split けんかを:
- ✕ けんかたまにをしました – incorrect.

Your example:

たまに双子の妹とけんかをしました。
is perfectly natural and probably the most common alternative ordering.

Why is the verb in the polite past form (しました)? Could I say けんかした instead?

しました is the polite past of する.
した is the plain past of する.

They both mean “did (fight)” in terms of time; the difference is only the politeness level:

けんかをしました。 – polite; used in:
- talking to teachers, strangers, in formal writing, etc.
けんかした。 – plain; used:
- with friends, family, diaries, casual narration, etc.

So you could say:

高校のとき、双子の妹とたまにけんかした。
→ Same meaning, just casual/plain style.

Choose しました or した depending on who you’re talking to and the overall style of your speech or writing.