Learn Hardcore Chinese: yǒuxiē xuéshēng xiàkè yǐhòu qù gōngyuán sànbù, yǒuxiē xuéshēng huíjiā xiūxi. - Some students go for a walk in the park after class; some students go home to rest.

一些学生 – also means some students.
- Often a bit more neutral; can sound slightly more like “a number of / several”.
- Here, 一些学生下课以后去公园散步 would be fine and natural.
有的学生 – literally “there are students who…”.
- Very commonly used in a paired pattern: 有的……，有的…… = “some…, (while) others…”.
- So you could also say:
  有的学生下课以后去公园散步，有的学生回家休息。
  This is very idiomatic.

Subtle feel:

有些 and 一些: “some (number of)”.
有的: “there are those who…”, especially good when contrasting two groups.

All three are grammatical here; the choice is mostly about style and nuance.

Why is there no measure word between 有些 and 学生? Should it be 有些个学生?

With 一些 / 有些 / 有的 / 很多 / 许多, Chinese normally does not use a measure word before a noun of people or things:

一些学生 (not 一些个学生)
有些人 (not 有些个人)
很多老师
许多问题

Measure words (like 个, 只, 杯) are mainly used after specific numbers:

三个学生
五个老师
两只猫
三杯咖啡

So 有些学生 is the natural pattern:
[indefinite quantity word] + [noun], no classifier needed.

Can we omit the second 学生 and just say: 有些学生下课以后去公园散步，有些回家休息?

Yes, that’s grammatical and natural:

有些学生下课以后去公园散步，有些回家休息。

In the second clause, 学生 is omitted because it is understood from context. Native speakers often drop repeated nouns like this to avoid redundancy.

However:

Keeping 学生 (有些学生回家休息) sounds a bit clearer and more formal.
Omitting it (有些回家休息) sounds more conversational and a bit lighter.

Both are correct; choice depends on style and emphasis.

Why is the time phrase 下课以后 placed after 学生? Could I move it to a different position?

The given order:

有些学生下课以后去公园散步

follows a very common pattern in Chinese:

Subject + Time + Place + Verb + (other information)
有些学生 + 下课以后 + 去公园 + 散步

Possible positions for 下课以后:

Before the subject:
- 下课以后，有些学生去公园散步，有些学生回家休息。
  Very natural. It sets “after class” as the time frame for the whole sentence.
After the subject (as in the original):
- 有些学生下课以后去公园散步……
  Also very natural; 有些学生 is mentioned first as the topic.

Positions that are unnatural/wrong:

✗ 有些下课以后学生去公园散步 – incorrect word order.
✗ 有些学生去公园下课以后散步 – 下课以后 should modify the time, not be placed after the place phrase this way.

So both:

下课以后，有些学生……
有些学生下课以后……

are good; they just differ in what you present first (time or people).

What is the difference between 下课 and 放学? Could I use 放学以后 here instead?

Both relate to the end of classes, but they are not identical:

下课 – “class is over”, the end of a specific class period.
- After one lesson finishes, but the school day might continue.
- 下课以后去喝水。 – “After the class ends, (they) go drink water.”
放学 – “school is over”, the end of the school day.
- Typically when students leave school to go home.
- 放学以后回家。 – “After school (is over for the day), (they) go home.”

In your sentence:

下课以后去公园散步 – after a class finishes, some students go for a walk.
You could also say 放学以后去公园散步, but then it means after the whole school day, which is a slightly different time point.

Can I say 下课后 instead of 下课以后? Is there any difference between 以后 and 后?

Yes, you can say:

下课后有些学生去公园散步，有些学生回家休息。

以后 vs 后:

以后 – literally “afterwards / in the future”, a bit longer; very common.
后 – shorter, slightly more formal/concise in some contexts.

When attached directly to a time or event:

下课后 ≈ 下课以后
吃饭后 ≈ 吃饭以后
三年后 ≈ 三年以后

In everyday speech, 以后 may feel just a bit more casual, but in this usage the difference is small; both are fine.

Why is there no 他们 before 去 and 回家? In English we say “they go…”.

Chinese often omits pronouns (like 他 / 她 / 他们) when the subject is already clear from context.

In this sentence:

The subject 有些学生 is clearly stated at the beginning of each clause.
So Chinese does not need:
- ✗ 有些学生下课以后他们去公园散步
- ✗ 有些学生他们回家休息

That would sound redundant or awkward.

Structure:

有些学生下课以后去公园散步
Subject = 有些学生
有些学生回家休息
Subject = 有些学生

Once the subject is known, Chinese just continues with the verbs.

Why is there a comma ， between the two parts instead of using 和 (“and”)?

You have two separate clauses describing two different groups:

有些学生下课以后去公园散步，
有些学生回家休息。

They are contrasting:

some do X,
(while) some do Y.

Chinese usually joins such parallel/contrasting clauses with:

a comma ，
sometimes with a conjunction like 而 for contrast:
- 有些学生……，而有些学生……

Using 和 here would be odd, because 和 mainly links:

nouns:
- 老师和学生 – “teachers and students”
sometimes verbs / verb phrases that share the same subject.

But here, the clauses have their own subjects (each has 有些学生), so a comma is the right connector.

What is the function of 去 in 去公园散步? Could I say 在公园散步 instead?

去 means “to go (to)” and expresses movement toward a place:

去公园散步 – “go to the park to take a walk”

Structure:

去 + [place] + [do something]

So:

去公园散步
去中国留学
去朋友家吃饭

If you say 在公园散步, that focuses on where the action happens, not the movement:

在公园散步 – “take a walk in the park” (location)
去公园散步 – “go (to the park) to take a walk” (movement + action)

Both are grammatical, but:

In your sentence, 去公园散步 emphasizes students going somewhere after class.
在公园散步 would lose the idea of “going there” and just state the place of the walk.

Is 散步 a verb, a noun, or a verb-object combination? How is it used?

散步 is usually treated as a verb meaning “to take a walk / to go for a walk”. It is a verb-object type word, but it behaves like a single verb in most cases.

Typical uses:

我晚上常常散步。 – I often take a walk in the evening.
他们在公园散步。 – They are taking a walk in the park.
我们去河边散步吧。 – Let’s go to the river and take a walk.

You usually do not add an extra object after 散步:

✗ 散步路 – unnatural
✔ 在路上散步 – “take a walk on the road”

So in your sentence:

去公园散步 = “go to the park (and) take a walk”

Why is there no “to” marker before 散步 or 休息, like “go to take a walk / go home to rest”?

Chinese does not need a particle like English “to” to link verbs. It often simply puts verbs in sequence:

去公园散步 – literally “go park walk”
- understood as “go to the park to take a walk”
回家休息 – literally “return home rest”
- understood as “go home to rest”

General pattern:

Verb 1 + Place + Verb 2

Examples:

去朋友家吃饭 – go to a friend’s place to eat
上街买东西 – go to the street (downtown) to buy things
回宿舍睡觉 – go back to the dorm to sleep

English uses “to / in order to”; Chinese just juxtaposes the verbs and the relationship (purpose) is understood from context.

How do we know if this sentence is about the past, present, or future, since there is no tense marker?

Chinese verbs do not change form for tense. Time is shown by:

time words (昨天, 明天, 现在, 下课以后, etc.)
context
aspect particles like 了, 过, 着 (but these show completion, experience, etc., not simple tense like in English)

Your sentence:

有些学生下课以后去公园散步，有些学生回家休息。

By itself, it could mean:

a general habit: “Some students (usually) go for a walk after class…”
a scheduled plan: “Some students will go for a walk after class…”

If you want to make it clearly past, you can add markers:

昨天有些学生下课以后去了公园散步，有些学生回家休息了。

If you want to make it clearly future, you can add 会 or a future time word:

明天有些学生下课以后会去公园散步，有些学生会回家休息。

Does 回家休息 literally mean “rest at home”? Why is the order not 在家休息?

回家休息 literally: “go back home and rest”.
- Emphasis on the movement back home, then the action of resting.
在家休息 literally: “rest at home”.
- Emphasis on where the resting happens (location), not the movement.

Your sentence uses:

有些学生回家休息。 – “Some students go home to rest.”

This contrasts nicely with:

有些学生下课以后去公园散步。 – “Some students go to the park to walk.”

Both verbs 去 (“go”) and 回 (“return”) highlight movement to/from a place, fitting the pattern:

[go somewhere] + [do something there]

Could I start the whole sentence with 下课以后? Would that change the meaning?

Yes, you can say:

下课以后，有些学生去公园散步，有些学生回家休息。

This is completely natural and very common.

Difference in feel:

有些学生下课以后去公园散步……
- Topic starts with “some students” and then adds when and what they do.
下课以后，有些学生去公园散步，有些学生回家休息。
- Topic starts with “after class” (the time frame), then explains what different students do.

The basic meaning is the same. The change is more about what you highlight first: the people or the time.