Cuando el semáforo se puso en verde, el coche de detrás tocó el claxon.

In Spanish, ponerse is often used to mean to become or to change into a state.

So:

• ponerse rojo = to turn red
• ponerse nervioso = to become nervous
• ponerse en verde = to turn green

With traffic lights, ponerse en + colour is very natural in Spain. Here, el semáforo se puso en verde means the light changed to green.

The se is part of the verb ponerse. It is not reflexive in the literal English sense of to put oneself here; it is just how this verb works when it means to become.

With traffic lights and similar signals, Spanish often uses ponerse en + colour:

• ponerse en rojo
• ponerse en ámbar
• ponerse en verde

This sounds more idiomatic for lights changing colour.

By contrast, with people or things becoming a colour, Spanish often uses the adjective directly:

• Se puso rojo = He/She turned red
• Las hojas se pusieron amarillas = The leaves turned yellow

So en verde is especially natural here because we are talking about a traffic light.

Both verbs describe completed actions/events in the past:

• se puso en verde = it turned green
• tocó el claxon = the driver honked the horn

The preterite is used because these are seen as single, finished events.

This sentence is basically a sequence:

1. The light turned green.
2. The car behind honked.

If you used the imperfect, it would sound like background description or an ongoing action, which is not the main idea here.

Because cuando is introducing something that actually happened as a completed event in the past.

• Cuando el semáforo se puso en verde... = When the traffic light turned green...

Since the light turning green is a specific event, the preterite fits well.

Compare:

• Cuando llegó, salimos. = When he/she arrived, we left.
• Cuando era niño, iba mucho al parque. = When I was a child, I used to go to the park a lot.

In the second example, cuando introduces a habitual/background situation, so the imperfect is used. In your sentence, it is a one-time event, so the preterite is used.

De detrás means something like from behind / the one behind. In this sentence:

• el coche de detrás = the car behind / the car at the back

It is a very natural way in Spanish to identify something by its position relative to another thing.

You can think of it as shorthand for:

• el coche que estaba detrás = the car that was behind

Spanish often uses this structure:

• la puerta de arriba = the upstairs door / the top door
• el vecino de abajo = the downstairs neighbour
• el coche de delante = the car in front
• el coche de detrás = the car behind

Both relate to behind/back, but they are not always interchangeable.

• detrás usually means behind something
• atrás often means back/backwards/at the back

In this sentence, de detrás is the natural choice because it identifies the car as the one behind.

For example:

• El coche de detrás = the car behind
• Mira atrás = look back
• Da un paso atrás = take a step back

So detrás is more clearly about position relative to something else.

Yes, literally tocar means to touch, but in Spanish it also means to play an instrument or to sound certain devices.

So:

• tocar el piano = to play the piano
• tocar el timbre = to ring the doorbell
• tocar el claxon = to honk the horn / sound the horn

This is a normal Spanish expression. English uses honk, while Spanish often uses tocar with claxon.

In everyday Spanish, you may also hear:

• pitar = to honk
• dar un pitido = to beep / give a honk

But tocar el claxon is perfectly correct and natural.

Because in Spanish, when talking about body parts, clothing, vehicle parts, and many familiar objects, the definite article is often used where English might use a possessive or no article.

So Spanish says:

• tocó el claxon = honked the horn
• me lavé las manos = I washed my hands
• cerró la puerta = he/she closed the door

Spanish prefers the horn here rather than his horn unless you specifically need to emphasize whose horn it was.

Yes, claxon is a normal word in Spain for a car horn.

Other Spanish-speaking regions may prefer different words, such as:

• bocina
• corneta in some places

But for Spanish from Spain, claxon is very familiar and appropriate.

You may also hear people express the action with a different verb:

• pitó = honked
• dio un claxonazo = gave a loud honk
• tocó el claxon = sounded the horn

All of these can be natural depending on style and context.

Because Spanish usually does not need subject pronouns when the verb ending already shows the subject.

• tocó already tells you it is he/she/it in the preterite.

Here the subject is explicitly given as el coche de detrás, so adding él would be unnecessary.

Spanish often leaves out pronouns unless they are needed for:

• emphasis
• contrast
• clarity

So this is completely normal:

• el coche de detrás tocó el claxon

not:

• él el coche de detrás tocó el claxon

Yes. The original sentence is natural, but there are several other ways a native speaker might say something similar in Spain:

• Cuando el semáforo se puso en verde, el coche de atrás pitó.
• Cuando se puso verde el semáforo, el coche de detrás tocó el claxon.
• En cuanto el semáforo se puso en verde, el coche de detrás tocó el claxon.

These versions change style slightly:

• de atrás instead of de detrás: also common
• pitó instead of tocó el claxon: more compact
• en cuanto instead of cuando: closer to as soon as

But your original sentence is fully idiomatic.