Learn Hardcore Chinese: wèile jiéshěng néngyuán, xuéxiào wǎnshang shí diǎn yǐhòu guān diào hěnduō dēng. - To save energy, the school turns off many lights after ten o’clock at night.

Can you leave it out?
- 节省能源，学校晚上十点以后关掉很多灯。
  This is not wrong, but it sounds more like informal shorthand and a bit abrupt.
- With 为了, it’s clearer and more natural that this is the purpose of what follows.
Can you move it?
These are also natural:
- 学校为了节省能源，晚上十点以后关掉很多灯。
- 学校晚上十点以后为了节省能源关掉很多灯。 (OK but slightly heavier style)

All of them mean the same thing; putting 为了节省能源 at the very beginning is very common when you want to highlight the reason/purpose first.

You may also see 为节省能源 (without 了), which is more formal/concise in written Chinese. In everyday speech, 为了 is more common.

What’s the difference between 节省, 省, and 节约? Why use 节省 here?

All three can be translated as “to save / to economize”, but there are nuances:

省 (shěng)
- Very common and relatively colloquial.
- Examples: 省钱 (save money), 省电 (save electricity), 省时间 (save time).
- Often shorter and more casual.
节省 (jiéshěng)
- Slightly more formal and “proper”; emphasizes thrift / careful use.
- Often used with resources: 节省能源, 节省水, 节省开支.
- Sounds a bit more written or serious than just 省.
节约 (jiéyuē)
- Also formal; often used in slogans or official language.
- Very common in collocations like 节约用水, 节约能源, 节约资源.
- Has a “be frugal, don’t waste” feel.

In this sentence:

为了节省能源 and 为了节约能源 are both very natural.
为了省能源 is understandable, but sounds more casual and slightly less standard than 节省/节约 with 能源.

So 节省能源 matches the semi-formal, “policy-like” tone of what a school would say.

Is 能源 the same as 电 or 电力? When should I use each?

They are related but not the same:

能源 (néngyuán)
- Literally “energy sources / forms of energy”.
- A broad term including electricity, fuel, gas, oil, etc.
- Used in contexts like 节省/节约能源, 开发新能源 (develop new energy sources).
电 (diàn)
- “Electricity” in general.
- Used in everyday phrases: 没电了 (battery’s dead), 用电 (use electricity).
电力 (diànlì)
- “Electric power”, often a bit more technical or formal.
- E.g. 电力公司 (power company), 电力供应 (power supply).

In this sentence, the school really wants to save electricity by turning off lights, but it’s expressed more generally and formally as 节省能源, which sounds more like an environmental / policy statement.

You could say:

为了节约用电，学校晚上十点以后关掉很多灯。
(More specific: “in order to save electricity…”)

Why is 晚上十点以后 placed where it is? Can I move it, or say 在晚上十点以后 instead?

Chinese prefers this basic order:

Subject + (Time) + Verb + Object

So:

学校 + 晚上十点以后 + 关掉很多灯
= “The school + after 10 p.m. + turns off many lights.”

This is perfectly standard.

You can move or slightly change the time expression:

Fronting the time:
- 晚上十点以后，学校关掉很多灯。
  Also very natural; it emphasizes the time.
Adding 在:
- 学校在晚上十点以后关掉很多灯。
  Also natural. 在 here is like “at / during (a time)”.

Where it becomes unnatural is:

✗ 学校关掉很多灯晚上十点以后。
Time phrases usually do not go after the main verb phrase like this.

What does 掉 add after 关? What’s the difference between 关灯 and 关掉灯?

掉 (diào) here is a resultative complement that indicates a completed, often final or “removed” result:

关灯 → “turn off the light(s)” (simple action, focus on the action itself)
关掉灯 → “turn the light(s) off completely; end up with them off”

In many contexts, both are possible:

老师让我们把灯关了。
老师让我们把灯关掉。

The version with 掉:

Emphasizes the result more strongly: the lights end up off.
Often sounds slightly more vivid or colloquial.

In your sentence:

关掉很多灯 is very natural.
关很多灯 is also understandable, but 关掉很多灯 sounds a bit smoother and more typical for this kind of statement.

You’ll see 掉 used similarly with other verbs:

吃掉 (eat up, finish eating)
用掉 (use up)
删掉 (delete/remove completely)

It frequently implies that something is removed / all gone / finished.

Why is it 很多灯 without a measure word? Could I say 很多盏灯 or 很多的灯 instead?

In Chinese, the usual pattern is:

Number/quantity + Measure word + Noun

But certain quantifiers (including 很多) can directly modify a noun without a measure word in everyday usage:

很多书
很多人
很多灯

So 很多灯 is completely natural.

With a measure word
You can also say:
- 很多盏灯 (盏 is a specific measure for lamps/lights)
- 很多个灯 (generic 个, also heard in speech)
These sound a bit more specific/“counted”, but the meaning is almost the same in this context.
With 的
- 很多灯 → most common and smooth.
- 很多的灯 → grammatically OK, but adds a tiny bit of emphasis on “many”, and sounds less natural here.

In short: 很多灯 is the most typical choice; 很多盏灯 is fine if you want to be more precise or formal.

What exactly does 以后 mean here? Is it “after 10 p.m.” or “from 10 p.m. and later”?

In 晚上十点以后, 以后 means “after / later than” that time.

In everyday usage, it often works like “after X (including around that point and later)” rather than a mathematically sharp “strictly greater than X”.

晚上十点以后 ≈ “after 10 p.m.” / “10 p.m. and later”.

Some related patterns:

十点以前 → before 10
十点以后 → after 10
从十点以后 → starting from 10 and after (a bit redundant, but used)
从十点开始 → starting at 10

So the sentence means that starting from 10 p.m.-ish onward, the school turns off many lights. It expresses a time period boundary, not a one-time event.

How do we know if this sentence describes a habit/rule or a one-time action, since there’s no tense marker like 了?

Chinese doesn’t mark tense the way English does; it relies on:

Context
Time expressions
Aspect particles like 了, 过, 着

In this sentence:

There is no 了.
The time phrase 晚上十点以后 describes a repeated time period, not a specific date.
The subject is 学校, an organization that can have rules or policies.

So the most natural reading is:

“As a routine rule, the school turns off many lights after 10 p.m. (regularly / every day).”

If it were describing a specific future plan, one might say:

为了节省能源，学校晚上十点以后会关掉很多灯。
(will turn off many lights…)

For a specific past action, you might see:

昨天晚上十点以后，学校关掉了很多灯。
(added 昨天 and 了).

Could we omit 晚上 and just say 十点以后? Would the meaning change?

Yes:

为了节省能源，学校十点以后关掉很多灯。

is grammatically correct and would usually be understood as “after 10 o’clock”.

However:

十点以后 is ambiguous between 10 a.m. and 10 p.m. if there’s no context.
晚上十点以后 clearly specifies 10 p.m..

So 晚上 just makes the time more precise. In a context where it’s already obvious you’re talking about nighttime, 晚上 could be omitted without confusion.

Is it natural to use 学校 as the subject here? Shouldn’t it be “they” or “the staff”?

Using 学校 as the subject is very natural in Chinese.

Organizations, institutions, and places commonly act as subjects to represent their management or staff:

学校要求学生穿校服。
“The school requires students to wear uniforms.”
公司提供午餐。
“The company provides lunch.”
酒店不允许吸烟。
“The hotel doesn’t allow smoking.”

So 学校晚上十点以后关掉很多灯 is fully natural and understood as “The school (meaning the school administration/staff) turns off many lights after 10 p.m.”

There’s no need to insert an explicit “they”.

Can I rewrite the sentence as 学校为了节省能源晚上十点以后关掉很多灯? Is that better or worse than putting 为了节省能源 at the very beginning?

Yes, this version is also grammatical:

学校为了节省能源，晚上十点以后关掉很多灯。

Comparing:

为了节省能源，学校晚上十点以后关掉很多灯。
- Emphasizes the purpose first.
- Very typical in explanatory or policy-like language.
学校为了节省能源，晚上十点以后关掉很多灯。
- Emphasizes the school first, then gives the purpose.
- Also very natural and common.

Both are fine. The difference is mostly about information flow / emphasis, not grammar.

Could I say 学校晚上十点以后把很多灯关掉 instead? What’s the difference between that and 关掉很多灯?

Yes, you can use a 把-construction here:

学校晚上十点以后把很多灯关掉。

Compare the two:

学校晚上十点以后关掉很多灯。
- Simple S–V–O order.
- Neutral, straightforward description.
学校晚上十点以后把很多灯关掉。
- Uses 把 to foreground the object + its resulting state.
- Slightly stronger focus on “those many lights end up being turned off”.

In this particular sentence, the difference is fairly subtle, and both are acceptable. If you later wanted to add another result or follow-up clause, 把 can make the structure clearer, e.g.:

学校晚上十点以后把很多灯关掉，只留走廊的灯。
“After 10 p.m. the school turns off many lights, leaving only the corridor lights.”

But as a standalone sentence, 关掉很多灯 (without 把) is perfectly natural and maybe a bit simpler.