Learn Hardcore Japanese: kodomotati ha natuyasumi ni supootu wo simasu. - Children play sports during summer vacation.

子供たちは: “As for the children,” “The children (on the other hand)…”
It sets 子供たち as the thing we’re talking about; then the rest of the sentence says something about them.

If you wrote 子供たちが夏休みにスポーツをします, then が would more strongly mark “the children” as the grammatical subject and can sound a bit like “(it is) the children (who) play sports in summer vacation,” often with a nuance of contrast or new information.

In everyday neutral statements, X は … is very common to introduce the main topic.

Why is は pronounced “wa” instead of “ha” in this sentence?

When は is used as a particle (like the topic marker here), it is always pronounced “wa”, even though the kana is normally read “ha” in words.

As a syllable in a word:
- 花 (はな, “hana”) – “flower”
As the topic particle:
- 子供たちは … → pronounced kodomotachi wa

This is just a quirk preserved from historical Japanese spelling and is something you have to memorize:

Topic particle は → read wa
Direction particle へ → read e
Object particle を → read o (not “wo”)

Why is 夏休み followed by に? What does 夏休みに mean exactly?

に after a time expression marks a point in time.

夏休みに = “in/over/during summer vacation” (at that time period)

So:

夏休みにスポーツをします。
→ They do sports during summer vacation.

Contrast with other particles:

夏休みはスポーツをします。
- “As for summer vacation, (we) do sports.”
- This is making 夏休み the main topic, not just specifying time.
夏休みでスポーツをします。
- Much less natural; で is usually for locations (“at/in (a place)”), not times.

Time expressions often take に (e.g. 三時に, 来週に), but some frequent ones (like 今日, 明日, 毎日) can omit it. Here, 夏休みに is the natural choice.

Why is 子供たち used instead of just 子供? Doesn’t Japanese usually skip plurals?

Japanese often leaves singular/plural implicit, but it does have ways to show plurality when needed. たち is one of them.

子供 → “child” or “children” (depending on context)
子供たち → clearly “children,” “group of children”

たち:

Often attached to people (or animals treated like people):
- 私たち – we
- 友だちたち – friends (as a group)
Adds a sense of “plural / group / bunch of”, and sometimes a nuance of collectivity (a specific group you have in mind).

In this sentence, 子供たち emphasizes that we’re talking about children as a group. You could also say 子供は夏休みにスポーツをします, which would sound more like a general statement about children in general (“Children play sports during summer vacation”).

Is “summer vacation” the subject here because it comes right after 子供たち? How do we know who is doing the action?

The doer (subject) of the action is 子供たち (the children), even though が isn’t used.

Clues:

子供たちは – topic; usually also the logical subject in a simple sentence like this.
The only noun phrase that makes sense as the agent of “doing sports” is the children.
夏休みに is marked with に, which here is a time marker, not the subject marker.

So the structure is:

子供たちは – topic / (logical subject)
夏休みに – “during summer vacation” (time)
スポーツを – sports (object)
します – do

The fact that Japanese doesn’t always mark the subject with が is one reason learners rely heavily on particles and meaning to identify roles.

Why is it スポーツをします instead of just having a single verb like “to play sports”?

Japanese very often uses する (“to do”) with nouns to create a verb meaning “to do X.”

勉強 (study) → 勉強する – to study
運動 (exercise) → 運動する – to exercise
スポーツ (sports) → スポーツをする – to play/do sports

In スポーツをします:

スポーツ – the activity (a noun borrowed from English)
を – marks it as the object
します – polite form of する, “do”

This is a very standard pattern for many activities that are nouns in Japanese.

Is を necessary here? Can I say 子供たちは夏休みにスポーツします without を?

Yes, in casual spoken Japanese, を is often dropped when the meaning is clear:

スポーツをします → fully correct, neutral
スポーツします → very common in conversation

So:

子供たちは夏休みにスポーツをします。 – textbook/polite and fully explicit
子供たちは夏休みにスポーツします。 – also natural, especially in speech

In writing and in formal or educational contexts, it’s safer to include を.

Can I change the word order? For example, is 夏休みに子供たちはスポーツをします also correct?

Yes, Japanese word order is flexible, as long as the verb comes at the end and particles are attached correctly. These are all grammatical:

子供たちは夏休みにスポーツをします。
子供たちはスポーツを夏休みにします。 (grammatical but sounds a bit unnatural; we usually put the time earlier)
夏休みに子供たちはスポーツをします。 (also fine; slightly more emphasis on “in summer vacation”)

The most natural/default order is:

Topic (子供たちは)
Time (夏休みに)
Object (スポーツを)
Verb (します)

So the original sentence is the most typical, but changing the order mainly affects emphasis, not basic meaning.

What politeness level is します? How would this change in casual speech?

します is the polite (ます-form) of する.

Polite:
- スポーツをします。 – (They) play sports.
Casual/plain:
- スポーツをする。 – (They) play sports.

So for casual speech:

子供たちは夏休みにスポーツをする。

In more formal or honorific speech (talking about someone you respect greatly), you might see なさいます, but with スポーツ that would be rare and sound very stiff:

スポーツをなさいます。

For a general polite sentence as you might use with a teacher, in writing, or to someone you don’t know well, します is appropriate.

Does します here mean “do” now, or “will do,” or “usually do”? How do we know the tense/aspect?

Japanese non-past form (します / する) covers:

Habitual: “(They) usually do / (They) do (in general).”
Future: “(They) will do.”
Sometimes present if it’s clearly about something happening now.

In 子供たちは夏休みにスポーツをします。, the most natural reading is habitual or general fact:

“Children play sports during summer vacation.” / “The children usually play sports during summer vacation.”

To clearly show past, you change します to しました:

子供たちは夏休みにスポーツをしました。
→ The children played sports during summer vacation.

Context usually tells you whether a non-past verb is being used as general/habitual or future.