Learn Hardcore French: Quand le feu est rouge, nous nous arrêtons.

un feu (de signalisation) = a traffic light
le feu est rouge = the (traffic) light is red
la lumière means general “light” (sunlight, lamp light, brightness), not a traffic signal.

So Quand le feu est rouge is the natural way to say “When the (traffic) light is red” in French.

Why is it le feu (singular) and not les feux (plural)?

Here we’re talking about one specific traffic light in the situation, so French uses the singular: le feu.

You do see the plural in other expressions:

aux feux, au feu suivant = at the traffic lights, at the next light
Il y a des feux partout dans cette ville. = There are traffic lights everywhere in this city.

But in your sentence, the subject is one light at a time: le feu.

Why is it Quand le feu est rouge and not something like Quand est le feu rouge?

In French, a subordinate clause introduced by quand keeps normal statement word order: subject + verb.

Quand le feu est rouge… = When the light is red…
- subject: le feu
- verb: est

You only use inversion (verb + subject) for questions, not here.
Compare:

Quand le feu est rouge, nous nous arrêtons. = When the light is red, we stop.
Quand est-ce que le feu est rouge ? = When is the light red? (question)

Could I say Lorsque le feu est rouge instead of Quand le feu est rouge?

Yes. Lorsque and quand are very close in meaning in this type of sentence:

Quand le feu est rouge, nous nous arrêtons.
Lorsque le feu est rouge, nous nous arrêtons.

Both are correct and mean the same here.
Nuance:

lorsque is a bit more formal and written.
quand is more neutral and very common in speech and writing.

Functionally, you can treat them as interchangeable in this context.

Why is it the simple present: nous nous arrêtons, and not some other tense?

French uses the simple present for:

General truths and regular habits
- Quand le feu est rouge, nous nous arrêtons.
  = Whenever this situation happens, we (always) stop.
Actions happening now
- Nous nous arrêtons. = We are stopping / We stop.

So the French present covers both “we stop” and “we are stopping” in English.
Here it describes a regular rule of behavior, so the simple present is exactly right.

Why do we need the reflexive form nous nous arrêtons instead of just nous arrêtons?

In French there’s an important difference:

arrêter (non‑reflexive) = to stop something/someone
- Nous arrêtons la voiture. = We stop the car.
s’arrêter (reflexive) = to stop (oneself), to come to a stop
- Nous nous arrêtons. = We stop / We come to a stop (ourselves).

In your sentence, we are the ones who stop moving, so you must use the reflexive form:

Quand le feu est rouge, nous nous arrêtons.
→ We (ourselves) come to a stop when the light is red.

If you said Nous arrêtons, it would sound like “We stop [something/someone else].”

Why do we write s’arrêter with an apostrophe, and how does that become nous nous arrêtons?

The verb’s infinitive is s’arrêter because the reflexive pronoun se becomes s’ before a vowel:

se arrêter → s’arrêter (elision to avoid two vowels in a row)

To conjugate, you change se to match the subject:

je m’arrête
tu t’arrêtes
il/elle/on s’arrête
nous nous arrêtons
vous vous arrêtez
ils/elles s’arrêtent

With nous, se becomes nous, so:

nous + nous + arrêtons = nous nous arrêtons.

Why is it le feu est rouge and not something like le feu est en rouge?

With colors in French, you normally just use être + color adjective:

Le feu est rouge. = The light is red.
Le ciel est bleu. = The sky is blue.

The structure être en + color isn’t used to describe simple color states like this.
So:

✅ Le feu est rouge.
❌ Le feu est en rouge.

Why doesn’t rouge change form here? Shouldn’t it show masculine/feminine?

Rouge is an adjective that has the same spelling for masculine and feminine singular:

masculine singular: un feu rouge
feminine singular: une voiture rouge

Where it does change is in the plural:

masculine plural: des feux rouges
feminine plural: des voitures rouges

In le feu est rouge, feu is masculine and singular, and rouge already has the correct form, so nothing changes in the spelling.

How would this sentence sound in natural spoken French? Would people really say nous nous arrêtons?

Grammatically, nous nous arrêtons is perfectly correct and is common in writing and formal speech.

In everyday spoken French, many people would more naturally say:

Quand le feu est rouge, on s’arrête.

Here on means “we” in spoken French, and s’arrête is the matching reflexive form:

on s’arrête ≈ “we stop”

So:

Written / more formal: Quand le feu est rouge, nous nous arrêtons.
Everyday speech: Quand le feu est rouge, on s’arrête.

Are there any important pronunciation points in this sentence?

A few key points:

feu is pronounced roughly like “feu” [fø], with rounded lips, not like English “few.”
rouge is r is a guttural sound in the throat
- ou like oo in “food”
- final -ge like English “zh” in “measure.”
nous nous arrêtons has a required liaison:
- nous nous‿arrêtons → you hear nou-zou-zarrêtons.

Altogether:
Quand le feu est rouge, nous nous arrêtons.
≈ Kahn lə fø ɛ ʁuʒ, nu nu za-ʁe-tõ.