nà tiān gōngyuán lǐ jì yǒu niánqīng rén yòu yǒu lǎorén, huòxǔ shì yīnwèi tiānqì tèbié hǎo.

Chinese usually puts time expressions at or near the beginning of the sentence, before place and before the main action:

• 那天公园里既有… – That day, in the park, there were… (natural, very common)

You can move it, but it becomes less natural:

• 公园里那天既有年轻人又有老人 – grammatical, but sounds a bit clumsy.
• 那天，在公园里既有年轻人又有老人 – also OK; the comma adds a clear pause.

As for 在那天:

• 那天 by itself is normally used for “that day”.
• 在那天 / 在那一天 is possible, but sounds more formal or literary, and is less common in everyday speech.

So 那天 at the start is the most typical, natural choice for spoken and neutral written Chinese.

This is an existential sentence pattern:

[place] + 有 + [thing/person]
桌子上有一本书。– There is a book on the table.
公园里有很多人。– There are many people in the park.

In this pattern, the place-word (with a location suffix like 上, 里, 下, 旁边, etc.) itself serves as a locative; you don’t have to add 在:

• 桌子上有一本书。 ✔
• 在桌子上有一本书。 ✔ (also OK, just a bit more marked/emphatic)
• 公园里既有年轻人又有老人。 ✔
• 在公园里既有年轻人又有老人。 ✔ (also OK)

So 公园里 functions as “in the park”, with 里 indicating “inside/within”.
Adding 在 is allowed but not required in this pattern. The version without 在 is very common and sounds natural.

Here 有 expresses existence (“there is/there are”):

• 公园里有很多人。 – There are many people in the park.

Chinese uses:

• 有 for “there is/are” (existence)
• 是 for “X is Y” (equation/identification)

So:

• 公园里有很多年轻人。– There are many young people in the park. ✔
• 那些人是年轻人和老人。– Those people are young people and old people. ✔
• 公园里是很多年轻人和老人。 ✘ (unnatural; wrong structure)

In this sentence we’re not identifying what the park is; we’re stating what exists in the park. That’s why 有 is correct and 是 is not.

既…又… is a correlative structure meaning “both … and …” / “not only … but also …” with a fairly neutral to slightly formal tone.

Pattern:

• 既 A 又 B

In this sentence:

• 既有年轻人又有老人 – There were both young people and old people.

A few points:

• With verbs or verb phrases:
  他既工作又学习。– He both works and studies.
• With adjectives:
  他既聪明又努力。– He is both smart and hardworking.
• With nouns, you often add something like 有 / 是:
  学校里既有中国学生又有外国学生。
  这道菜既是甜的又是辣的。

Comparison:

• 又…又… – similar meaning, slightly more colloquial:
  他又聪明又努力。
• 不但…而且… – often carries a stronger “not only but also” emphasis:
  他不但聪明，而且很努力。

Here 既…又… simply highlights that both groups (young people, old people) are present, with a neat, balanced feel.

Chinese tends to like parallel and balanced patterns in these correlative constructions. So:

• 既有年轻人又有老人
  → [既 + 有 + 年轻人] + [又 + 有 + 老人]

Both halves have the same structure: 又 + 有 + noun. This sounds smooth and natural.

If you say:

• 既有年轻人又老人

it is not ungrammatical, but it feels incomplete/less natural, because the second half loses the verb 有 and breaks the parallelism. Native speakers almost always repeat 有 here.

1. No 的

In many common combinations, an adjective directly forms a compound noun with a basic noun, without 的:

• 年轻人 – young person / young people
• 老人 – old person / elderly people
• 男人 – man/men
• 女人 – woman/women

Using 的 here (年轻的人, 老的人) is either:

• more descriptive or contrastive, or
• simply sounds unnatural (老的人 is rarely used; 老人 is the normal word).

So 年轻人, 老人 are standard nouns, not just “adjective + noun” phrases.

2. No 们

Chinese does not require plural marking the way English does. 人 can mean “person” or “people” depending on context:

• 一个人 – one person
• 很多人 – many people
• 公园里有年轻人和老人 – there are young people and old people in the park

们 is mostly used for:

• personal pronouns: 我们, 你们, 他们
• some human nouns when the individuality is important: 学生们, 孩子们 (often in writing or when emphasizing “the students as a group”).

Here we’re talking about groups in general; no need for 们.

Chinese doesn’t have tense like English. It often relies on time words and context instead of verb changes.

Here, 那天 (“that day”) clearly sets a past time frame, so no extra marker is needed.

Why no 了?

• 了 often marks completion or change of state.
• In existential descriptions like this, we’re just describing the state on that day:
  那天公园里既有年轻人又有老人。
  → On that day, (the situation was that) there were both young and old people in the park.

If you said:

• 那天公园里有了很多人。

you’re emphasizing a change (“there came to be many people”), which changes the nuance.

So the sentence without 了 is the natural way to describe the state of affairs on that day. The past meaning comes from 那天, not from verb morphology.

或许 (huòxǔ) is an adverb meaning “perhaps / maybe”.

In the sentence:

• 或许是因为天气特别好。
  → Perhaps (it was) because the weather was especially good.

Comparison:

• 或许 – “perhaps”; slightly softer or more literary in feel, but common in speech too.
• 也许 – also “perhaps”; very close in use to 或许, often interchangeable:
  也许是因为天气特别好。
• 可能 – “possibly / might / may”, often feels a bit more objective or “probable”:
  可能是因为天气特别好。– It’s possible / likely that it was because the weather was especially good.

All three work here; the choice mostly affects nuance and style, not grammar.

The structure here mirrors English:

• 或许是因为天气特别好。
  → Perhaps it’s because the weather was especially good.

Think of it as:

• (原因) 是 因为天气特别好。 – (The reason) is that the weather was especially good.

是 links the (implied) subject “(the reason)” with the reason clause.

Can we drop 是?

• 或许因为天气特别好。 – This is also grammatical, but it sounds a bit more casual and less complete; it’s like saying “Maybe because the weather was especially good” and leaving the “it’s” understood.
• 或许是因为天气特别好。 – Sounds more natural and idiomatic in a full sentence, especially after a statement you’re explaining.

So you can omit 是, but including it is very typical and slightly more fluent in this pattern.

There are two main uses of 因为:

1. Conjunction in a paired pattern:

   • 因为…，所以… – because…, therefore…
     因为天气很好，所以公园里有很多人。

2. Preposition-like “because of” introducing a reason phrase:

   • 因为天气很好 – because the weather was good / because of the good weather

In our sentence:

• 或许是因为天气特别好。

Here, 因为天气特别好 is a reason phrase (“because the weather was especially good”). The whole thing works like:

• Perhaps (it was) because of the especially good weather.

We’re not using the full 因为…所以… pattern; instead, we first state a fact, then offer a possible reason:

• 那天公园里既有年轻人又有老人，
  或许是因为天气特别好。

So 因为 stands alone perfectly well here; 所以 is not required.

All three mean the weather is good, but the nuances differ:

• 很好 – “very good” / “good”; fairly neutral intensity.
• 非常好 – “extremely/very good”; stronger than 很好, often just “very, very good”.
• 特别好 – literally “especially good”; often implies:
  • unusually good, better than usual / better than expected, or
  • good in a special way that stands out.

For weather:

• 天气很好。– The weather is very good. (normal positive comment)
• 天气非常好。– The weather is extremely good. (strong praise)
• 天气特别好。 – The weather is especially good (today/that day in particular; stands out).

In the sentence, 特别好 helps explain why both young and old went to the park: the weather was exceptionally nice, not just “fine”.

Yes, you can reverse the order:

• 或许是因为天气特别好，那天公园里既有年轻人又有老人。

This means essentially the same thing, but the focus/order of presentation changes:

1. Original:

   • 那天公园里既有年轻人又有老人，或许是因为天气特别好。
     → First describe what you saw; then suggest a possible reason.

2. Reversed:

   • 或许是因为天气特别好，那天公园里既有年轻人又有老人。
     → First propose a reason; then state the resulting situation.

Both are grammatical and natural. The original order (fact → possible reason) is very common when narrating or describing a scene that you then explain.