Learn Hardcore Spanish (Latin America): Cuando el semáforo está en rojo, el conductor detiene su carro. - When the traffic light is red, the driver stops his car.

cuando is the conjunction for “when,” introducing a time clause. When a subordinate clause (here, the cuando-clause) comes before the main clause, Spanish style recommends a comma to signal the break, though it isn’t absolutely mandatory. Putting the time clause first emphasizes the condition (“when the light is red”) before stating the action.

Why are both verbs (está and detiene) in the present tense, even though the sentence refers to future or potential stopping?

Spanish often uses the present indicative for general truths, habitual actions, or instructions. In English we say “When the light is red, the driver stops…” to express a rule. In Spanish, the present tense serves the same purpose, even if each instance might occur in the future.

Why is está used instead of es to describe the traffic light’s color?

estar describes temporary states or conditions, and traffic lights change colors. ser would suggest a permanent characteristic, which doesn’t fit here. Thus we say el semáforo está en rojo (“the light is in red [state]”).

Why do we say en rojo instead of just rojo?

With traffic lights, Spanish typically uses the construction estar + en + color to indicate the current signal state (“in red,” “in green,” etc.). Saying está en rojo makes it clear you’re talking about the lamp showing red, not describing the traffic light as inherently red.

What is the function of the definite article el before semáforo?

When talking about things in a general or habitual sense (“the red light in all intersections”), Spanish uses the definite article. It’s not a specific, single traffic light, but the concept of “the traffic light.” Therefore, el semáforo is correct.

Why is the verb detiene followed directly by su carro without a preposition like a?

detener is a transitive verb requiring a direct object. Cars are inanimate, so there’s no personal “a” (that only appears before specific people or personified animals). You simply say detiene su carro.

What’s the difference between detener and parar in this context? Could we say para su carro instead?

Both verbs mean “to stop,” but:

detener is transitive and a bit more formal—“bring something to a halt.”
parar can be transitive or intransitive and is more general.
You could say para su carro, but you often hear detiene su carro in written or formal contexts. Colloquially, drivers might say para el carro or simply se para.

Why is the possessive su used for carro, and how do we know it means “his” and not something else?

su is the third-person possessive adjective (“his,” “her,” “your” formal, or “their”). Here it refers back to el conductor (“the driver”), so it means “his car.” Context tells you whose car it is (the driver’s).

Why is carro chosen instead of coche or auto?

Spanish vocabulary varies by region. In Latin America, carro is the most common word for a four-wheeled vehicle. In Spain, you’ll hear coche; in some countries, auto is also used. All mean “car,” but you pick the regional term.

Could we rewrite the sentence using a reflexive form, such as se detiene or se para?

Yes. Focusing on the car itself stopping, you can say:
Cuando el semáforo está en rojo, el carro se detiene.
or
… el carro se para.
Here detenerse or pararse makes the car the grammatical subject performing the action on itself. Both versions are natural.