Learn Hardcore Japanese: kodomotati ha natuyasumi ni umi de oyogimasu. - Children swim in the sea during summer vacation.

In conversation, は does not mean “subject” in the strict grammatical sense; it frames the topic and contrasts it with other possible topics (e.g., “The children (as opposed to adults) swim at the sea in summer vacation.”).

Why is は pronounced “wa” here and not “ha”?

The character は is normally read “ha” in words (like はな = hana, “flower/nose”), but when used as the topic particle, it is pronounced “wa”.

So:

As part of a word: は → ha
As topic particle: は → wa

This is just an irregularity of Japanese writing vs. pronunciation that learners have to memorize.

What does the たち in 子供たち mean?

たち is a suffix that often indicates a group or plural, especially for people.

子供 (こども) = child / children (can be singular or plural from context)
子供たち (こどもたち) = children (emphasizes that it’s a group)

It’s not a strict grammatical plural like -s in English; it’s more like “the child and their companions / group of children.”
You see it with other words too, like:

私たち (わたしたち) = we
先生たち (せんせいたち) = (the) teachers

Does 子供たち mean “the children” or just “children” in general?

Japanese does not mark definiteness (the/a) the way English does. 子供たち can mean:

the children (specific group, from context)
(some) children
children in general

In this sentence, context would decide. If the conversation is already about our kids, then it’s “the children.” If it’s a general statement, it can be “children” in general. The Japanese itself doesn’t force either reading.

What is the function of に in 夏休みに?

Here に is a time marker, indicating when something happens.

夏休み = summer vacation
夏休みに = during summer vacation / in summer vacation

So に can mark:

points in time: 三時に (at three o’clock)
days, periods, occasions: 日曜日に, 夏休みに

Often it can be translated as in, on, or at, depending on English idiom.

Why is で used after 海, and not に?

Both に and で can mark a place, but they have different roles:

に: where something exists or where you arrive (location/goal)
で: where an action takes place (location of activity)

In this sentence:

海で泳ぎます。 = They swim at/in the sea (the action of swimming happens there)

If you said 海に泳ぎます, it would sound strange, because 泳ぐ is an action; for the place of the action, Japanese uses で.

What is the basic/dictionary form of 泳ぎます, and what does the ます do?

The dictionary form is 泳ぐ (およぐ) = to swim.

泳ぎます is the polite non-past form:

泳ぐ → neutral/plain form
泳ぎます → polite form (used in most everyday polite speech)

The ます ending:

makes the verb polite
does not show past vs. future by itself; it just shows non-past (present/future).

So:

泳ぎます。 = (I/they) swim / (I/they) will swim – depending on context.

Is this sentence talking about the present, the future, or a habitual action?

The Japanese non-past form (泳ぎます) can cover:

future: The children will swim in the sea during summer vacation.
habitual: The children (usually) swim in the sea during summer vacation.

Japanese doesn’t distinguish these strictly like English. Context (e.g., “This year…” vs. “Every year…”) tells you whether it’s a specific future event or a general habit. The form itself just says “not past.”

Why is the word order 子供たちは夏休みに海で泳ぎます and not closer to English word order?

Japanese basic sentence structure is [Topic/Subject] – [Other Information] – Verb.
The verb normally comes at the end.

In this sentence:

子供たちは – topic: “the children”
夏休みに – when: “during summer vacation”
海で – where: “at the sea”
泳ぎます – verb: “swim”

So the rough pattern is:
[Who] [when] [where] [do what].

The order of the “middle” parts (time, place, etc.) is somewhat flexible, but the verb must stay at the end in standard statements.

Can some parts of this sentence be left out in natural Japanese?

Yes; Japanese often omits information that is obvious from context.

Depending on context, you could say:

子供たちは海で泳ぎます。
(Leaving out 夏休みに if the time is obvious or already mentioned.)
夏休みに海で泳ぎます。
(Omitting 子供たちは if we already know we’re talking about the children.)

The full sentence is perfectly correct and clear, but in natural conversation you often drop what’s already understood.

How do you read each part of 子供たちは夏休みに海で泳ぎます。 in romaji?

Word-by-word:

子供たち → kodomo-tachi
は (topic) → wa
夏休み → natsu-yasumi
に → ni
海 → umi
で → de
泳ぎます → oyogimasu

Altogether:
Kodomo-tachi wa natsu-yasumi ni umi de oyogimasu.

Is there a more casual way to say this sentence?

Yes, you can switch from polite ます to plain dictionary form:

Polite: 子供たちは夏休みに海で泳ぎます。
Casual/plain: 子供たちは夏休みに海で泳ぐ。

The meaning (non-past “swim/will swim”) is the same; only the politeness level changes.