Learn Hardcore Japanese: kodomotati ha kouen de genki ni asobimasu. - The children play energetically in the park.

たち is a suffix that pluralizes or collectivizes a noun. When you add it to 子供 (child), you get 子供たち, meaning “children.” It doesn’t always work with every noun, but with people-related words it’s very common.

Why is は used after 子供たち? What does it mark?

The particle は marks the topic of the sentence—the thing we’re talking about. Here, 子供たち is the topic (“As for the children…”). It sets up the rest of the sentence. Note that it’s not the same as the subject marker が, though often topics act like subjects.

What does で mean in 公園で?

The particle で indicates the location where an action takes place. So 公園で means “at the park” or “in the park,” specifying where the children play.

What’s the difference between で and に when you talk about location?

• で marks the place of an action (e.g. 公園で遊ぶ – play at the park).
• に marks a static location (e.g. 公園にいる – be at the park) or direction (e.g. 公園に行く – go to the park).

In other words, if something happens there, use で; if something exists or you move toward it, use に.

How is 元気に formed and what does it mean?

元気 is a na-adjective (or noun) meaning “healthy,” “lively,” or “energetic.” To turn it into an adverb that modifies a verb, you add に, giving 元気に, which means “energetically” or “vigorously.” Here it tells us how the children play.

What form is 遊びます, and how would you say this sentence in plain/informal style?

遊びます is the polite present/future form (the “-masu” form) of the verb 遊ぶ (to play). In plain (dictionary) form you’d say:
子供たちは公園で元気に遊ぶ。

How does the word order in Japanese compare to English in this sentence?

Japanese typically follows Topic–Time–Place–Object–Verb order. In our sentence:
• 子供たち　は (Topic)
• 公園　で (Place)
• 元気　に (Manner/adverb)
• 遊びます (Verb)
In English, we’d say “The children play energetically in the park,” where the verb usually comes before location and manner. Japanese always puts the verb at the end.