Learn Hardcore Russian: Не стоит включать свет ночью, если ребёнок уже спит. - It’s not worth turning on the light at night if the child is already sleeping.

Не стоит + infinitive is a common impersonal way to say it’s not worth / it’s not advisable to… In this structure, there is usually no explicit subject (no я/ты/мы). It’s like English You shouldn’t… or It’s better not to…, but expressed impersonally.

What does стоить mean here? Is it about money?

No. Although стоить can mean to cost, in (не) стоит + infinitive it means to be worth doing / to be advisable.
So Не стоит включать… = It’s not worth / not a good idea to turn on…

Why is включать used instead of включить?

включать is imperfective, which often fits general advice, repeated situations, or “in principle” statements:

Не стоит включать свет ночью… = general recommendation.

включить (perfective) would sound more like advice about a single specific instance:

Не стоит включить свет… is much less natural; if perfective is used, it’s usually in a clearer one-time context, but here the imperfective is the default.

What case is свет in, and why?

свет is in the accusative case because it’s the direct object of включать (to turn on what?):

включать (что?) свет

For inanimate masculine nouns, accusative often looks the same as nominative, so свет doesn’t change in form.

Why is ночью in the instrumental case form?

ночью is a fixed adverbial form meaning at night / during the night. Historically it’s related to instrumental, but as a learner you can treat ночью as an adverb meaning at night (like утром, днём, вечером).

Does если mean if or when here?

Here если is literally if, expressing a condition: if the child is already asleep.
If the speaker means it as a regular time situation (when the child is already asleep), Russian still often uses если in practical speech, but когда could also be used:

…когда ребёнок уже спит. = …when the child is already asleep.

Why is it ребёнок (nominative) and not some other case?

In the clause ребёнок уже спит, ребёнок is the grammatical subject of спит, so it’s nominative:

(кто?) ребёнок
- (что делает?) спит

What does уже add? Can I omit it?

уже means already and emphasizes that the child is currently in the sleeping state, so turning on the light would disturb them.
You can omit it:

…если ребёнок спит. This is still correct, just slightly less explicit than already asleep.

Is the word order flexible? Could it be Ночью не стоит включать свет…?

Yes, word order is flexible and changes emphasis:

Не стоит включать свет ночью… focuses first on the advice (not worth turning on…), then adds at night.
Ночью не стоит включать свет… foregrounds at night (as the setting/topic).

Both are natural.

How would this sound if I address someone directly (like You shouldn’t…)?

Russian often keeps it impersonal, but you can also use:

Тебе/вам не стоит включать свет ночью, если ребёнок уже спит. = You shouldn’t… (more direct)
Не включай(те) свет ночью, если ребёнок уже спит. = imperative: Don’t turn on the light… (stronger)

How is не стоит different from не надо or нельзя?

They differ in strength:

не стоит = gentle advice / not a good idea
не надо = don’t (advice/instruction), more direct
нельзя = not allowed / must not / impossible (strong prohibition)

So не стоит is the softest and most “recommendation-like.”

Any pronunciation/stress pitfalls in this sentence?

Common ones:

ребёнок has ё, stressed: рибЁнок (the ё sound is always stressed)
включАть stress on the last syllable: вклю-чАть
ночьЮ stress on ю: ночЮ
стОит stress on о: стОит