Learn Hardcore Chinese: xuéxiào yǒu yì cì huódòng, ràng xuéshēng xuéxí zěnme jiǎnshǎo lājī. - The school has an activity that lets students learn how to reduce trash.

学校有一次活动
literally: “At the school, there is one (time) activity.”
natural English: “The school has an activity” or “There will be an event at the school.”

Compare:

我有一本书。 – “I have a book.” (possessive 有)
桌子上有一本书。 – “There is a book on the table.” (existential 有)
学校有一次活动。 – “There is / will be an event at the school.” (existential 有)

Context (a notice, an announcement, etc.) tells us that this is about a future event, so we translate it as “will have” or “there will be.”

Why is 次 (cì) used as the measure word for 活动 (huódòng) instead of 个 (gè)?

次 is a measure word that emphasizes the number of times something happens. It’s used for actions, events, experiences:

一次活动 – “one time/activity/event”
去过一次中国 – “(have) been to China once”
见过他三次 – “have met him three times”

个 is a very general measure word and can be used for many nouns, including 活动, but it’s more about counting the thing, not the occurrence:

一个活动 – “one activity/event” (treating “activity” as an item)

In practice:

一次活动 focuses on this single occurrence of an event (this one happening).
一个活动 is also possible, but sounds a bit more like “one particular event/activity” as a “project” or “program.”

In announcements like this sentence, 一次活动 is very natural because schools often talk about “this one time/event we are organizing.”

What exactly does 让 (ràng) do in 让学生学习怎么减少垃圾? Does it mean “let,” “make,” or “have”?

让 here is a causative verb: it introduces who is caused/allowed/arranged to do something.

Pattern:

让 + person + verb phrase

Meanings depend on context and tone:

“let / allow”: 老师让我们早点回家。 – “The teacher let us go home early.”
“make / cause”: 雨让路很滑。 – “The rain makes the road slippery.”
“have (someone do something)” / “get (someone to do something)”:
- 公司让他出差。 – “The company had him go on a business trip.”

In your sentence:

让学生学习怎么减少垃圾
≈ “have the students learn how to reduce trash”
So the school organizes an event that causes / enables the students to learn.

A bit too literal but helpful:

“There is an event, [which] makes/lets the students learn how to reduce trash.”

So here 让 is close to English “have (someone) do something” or “let (someone) learn.”

Why do we say 学习 (xuéxí) instead of just 学 (xué) here? Are they different?

学 and 学习 are closely related; often they can both mean “to study / to learn.”

学 is shorter, slightly more casual.
学习 can sound a bit more formal or emphasize the process of studying.

In this sentence:

让学生学习怎么减少垃圾。

Using 学习 fits well because:

It’s describing a somewhat formal/school-related learning activity.
It echoes the idea of studying a topic rather than just “picking it up.”

You could say:

让学生学怎么减少垃圾。

This is grammatically fine and understandable, but 学习 sounds more natural in written/formal school announcements.

In many everyday cases:

学中文 / 学汉语 and 学习中文 / 学习汉语 can both be used.
学习 is just a bit more formal or textbook-like.

How does 怎么 (zěnme) work in 学习怎么减少垃圾? Is it like “how to” in English?

Yes. In this structure, 怎么 means “how” in the sense of “in what way / by what method.”

Pattern here:

verb + 怎么 + verb phrase

So:

学习怎么减少垃圾
→ “study how (to) reduce trash”

Chinese doesn’t need an extra “to” word like English does (“how to”). 怎么 + verb already means “how to (do something).”

Other examples:

教我怎么用这个软件。 – “Teach me how to use this software.”
你知道怎么去机场吗？ – “Do you know how to get to the airport?”

What’s the difference between 怎么减少垃圾, 如何减少垃圾, and 怎么样减少垃圾?

All three contain the idea of “how,” but they differ in formality and typical usage.

怎么减少垃圾
- Most natural and common in spoken, everyday Mandarin.
- Means “how to reduce trash,” “in what way can we reduce trash.”
如何减少垃圾
- More formal/literary.
- Common in written language: articles, news, essays, titles:
  - 如何减少垃圾？ – “How to reduce waste?” (article title)
- In speech, you’ll hear it too, but it sounds more formal than 怎么.
怎么样减少垃圾
- 怎么样 often means “how (is it)? / what do you think?” not “how to (do something).”
- E.g. 这个菜怎么样？ – “How is this dish?” / “What do you think of this dish?”
- Using 怎么样 directly before a verb phrase like 减少垃圾 is unusual.
  You’d more typically say:
  - 怎样减少垃圾？ (zěnyàng) – written/formal “how to reduce trash”
- 怎么 and 怎样 can both mean “how (to do something),” but 怎么 is more common in speech.

So for your sentence, 怎么减少垃圾 is the natural, conversational choice.
A more formal written version of the sentence might use 如何减少垃圾 instead.

What exactly does 减少 (jiǎnshǎo) mean? How is it different from just 减 (jiǎn) or 少 (shǎo)?

减少 is a verb meaning “to reduce / to decrease (the amount or number of something).”

减 by itself often means “to subtract / to cut / to reduce”, but is frequently used:
- in math: 5减2等于3 – “5 minus 2 equals 3”
- in collocations: 减肥 – “to lose weight,” 减价 – “to mark down (price)”
少 by itself is an adjective/adverb: “few / little / less.”
- 人很多，椅子太少。 – “There are many people; too few chairs.”
- 少吃点。 – “Eat less.”

减少垃圾 means:

“reduce the amount of trash” / “cut down on waste”

So:

减垃圾 sounds incomplete/awkward; it’s not the normal collocation.
少垃圾 would mean “less trash” as a description, not the action “to reduce trash.”

That’s why 减少垃圾 is the natural verb phrase for “reduce trash.”

How is 垃圾 pronounced and used? Is it countable in Chinese?

Pronunciation depends on region:

Mainland standard: lājī (both syllables neutral or second light)
Taiwan: commonly lèsè (different word, same characters)

In Mainland-style Mandarin you’ll usually learn:

垃圾 (lājī) – “trash / garbage / rubbish”

Usage:

It behaves like a mass noun (uncountable) in concept:
- 很多垃圾 – “a lot of trash”
If you want to “count” it, you count containers or units:
- 一袋垃圾 – a bag of trash
- 一桶垃圾 – a bin/bucket of trash
- 一车垃圾 – a truckload of trash
You can also say:
- 垃圾桶 – trash can
- 垃圾分类 – trash sorting / waste separation

So you normally don’t say “one trash, two trashes”; instead, you count bags, bins, pieces, etc., just like in natural English usage.

The English translation uses future tense (“there will be an activity”), but the Chinese sentence doesn’t show tense. How do we know it’s future, and how would we say it in the past?

Chinese doesn’t have verb endings showing tense like English -ed, will, etc.
Tense is usually understood from context, time words, and sometimes particles.

In:

学校有一次活动，让学生学习怎么减少垃圾。

there is no explicit time word, but in real life this kind of sentence is often used in:

an announcement
a plan
a description of something scheduled

So it’s naturally understood as:

“The school will have an event…” / “There is going to be an event…”

To make it clearly past, you could say:

学校有过一次活动，让学生学习怎么减少垃圾。
- 有过 emphasizes it happened before.
学校有一次活动，让学生学习怎么减少垃圾，那是上个月的事。
- Adding 上个月 (“last month”) clarifies time.
In many contexts, just:
- 学校有一次活动，让学生学习怎么减少垃圾。 with a past-time context (e.g. “Last year, …”) is enough for listeners to understand it as past.

So tense is mostly supplied by context; the basic structure itself doesn’t change much.

Could we change the sentence to 学校有一次让学生学习怎么减少垃圾的活动? Is that correct, and what’s the difference?

Yes, that version is correct and very natural.

学校有一次让学生学习怎么减少垃圾的活动。

Here, the part:

让学生学习怎么减少垃圾的

functions as an adjectival clause modifying 活动:

literally: “an activity that lets the students learn how to reduce trash.”

So:

Original:
学校有一次活动，让学生学习怎么减少垃圾。
→ “The school has an activity, (which) lets students learn how to reduce trash.”
(two clauses, with a pause/comma)
Modified:
学校有一次让学生学习怎么减少垃圾的活动。
→ “The school has an activity that lets students learn how to reduce trash.”
(one clause, with a 的-phrase describing the activity)

Meaning is almost the same; the 的 version makes the “learning how to reduce trash” more tightly attached as a defining feature of that activity. It also feels slightly more written/formal.

Could we also say 学校举行一次活动 instead of 学校有一次活动? What difference does 举行 (jǔxíng) make?

Yes, you can say:

学校举行一次活动，让学生学习怎么减少垃圾。

举行 means “to hold / to conduct (a formal event, meeting, ceremony, etc.)”.

Difference in nuance:

学校有一次活动…
- Neutral: “The school has/is having an event…”
- Focus on the existence of the event.
学校举行一次活动…
- “The school is holding an event…”
- Emphasizes that the school is actively organizing/hosting it.
- Sounds a bit more formal/official (announcements, news, written language).

So:

For a casual statement, 有一次活动 is perfectly fine.
For something like a school notice, news report, or formal description, 举行一次活动 is very common.