No pases el semáforo en rojo.

Because this sentence is a negative command: Don’t go past / don’t run the red light.

In Spanish, negative tú commands use the present subjunctive, not the normal present tense.

• pasas = you go past / you pass (statement)
• no pases = don’t go past / don’t pass (negative command)

So:

• Pasas el semáforo = You go past the traffic light
• No pases el semáforo = Don’t go past the traffic light

In Spanish, that is simply the normal rule for negative informal commands.

For tú:

• affirmative command: usually based on the imperative form
  
  • Pasa = Go past
• negative command: use no + present subjunctive
  
  • No pases = Don’t go past

This happens with many verbs:

• Habla / No hables
• Come / No comas
• Escribe / No escribas

So No pases follows a very common pattern.

It comes from the present subjunctive of pasar.

Steps:

1. Start with the yo form of the present tense: paso
2. Remove the -o
3. Add the subjunctive endings

For pasar, the present subjunctive is:

• yo: pase
• tú: pases
• él/ella/usted: pase
• nosotros: pasemos
• vosotros: paséis
• ellos/ellas/ustedes: pasen

So no pases is the tú negative command form.

Here, pasar el semáforo means to go past the traffic light.

In context, No pases el semáforo en rojo means:

• Don’t go past the traffic light when it’s red
• more naturally in English: Don’t go through the red light or Don’t run the red light

Spanish often expresses this idea with pasar rather than using exactly the same wording as English.

Because semáforo is a masculine noun in Spanish.

So you say:

• el semáforo
• un semáforo
• los semáforos

Even though it ends in -o, this is actually the normal masculine pattern, so it fits what learners often expect.

Literally, en rojo means in red.

With traffic lights, Spanish commonly says:

• estar en rojo = to be red
• estar en verde = to be green
• estar en ámbar = to be amber

So el semáforo en rojo means the traffic light when it is red or the traffic light on red.

In natural English, we usually just say the red light.

It can, but semáforo is very common because it clearly refers to the traffic signal itself.

Compare:

• No pases el semáforo en rojo = Don’t go through the red light
• No cruces con la luz roja = Don’t cross on red

Spanish often prefers talking about the traffic light (semáforo) rather than only the red light as English does.

Not usually. Cruzar means to cross, so it does not fit as naturally with semáforo in this structure.

More natural options would be:

• No pases el semáforo en rojo
• No te saltes el semáforo en rojo
• No cruces con el semáforo en rojo (possible in some contexts, but different structure)

So if you want the exact idea of don’t go past / don’t run the red light, pasar or saltarse is more natural than cruzar.

Both can be used, but they feel a bit different.

• pasar el semáforo en rojo = to go past the light when it’s red
• saltarse el semáforo en rojo = to skip / ignore / run the red light

Saltarse often sounds a bit stronger because it suggests breaking the rule more clearly.

For example:

• No pases el semáforo en rojo = Don’t go through the red light
• No te saltes el semáforo en rojo = Don’t run the red light / Don’t ignore the red light

Also note that saltarse is reflexive here:

• saltarse algo
• command: No te saltes...

Because Spanish usually omits subject pronouns when they are already clear from the verb form.

In No pases, the ending -es already shows that the command is for tú.

So Spanish normally says:

• No pases el semáforo en rojo

rather than:

• Tú no pases el semáforo en rojo

Adding tú is possible, but it usually adds emphasis, contrast, or emotion.

It is informal singular, used with tú.

• No pases = Don’t go past... (speaking to one person informally)

The formal singular version would be:

• No pase el semáforo en rojo (usted)

Other forms:

• No paséis el semáforo en rojo = informal plural in Spain (vosotros)
• No pasen el semáforo en rojo = formal plural, or plural in Latin America (ustedes)

Yes, it is understandable and natural enough in Spain.

In Spain, you may also hear:

• No pases en rojo
• No te saltes el semáforo en rojo
• No cruces en rojo

The exact phrasing depends on the situation:

• talking about a driver
• talking about a pedestrian
• giving a road-safety instruction

But No pases el semáforo en rojo is a very reasonable sentence for Don’t go through the red light.

The affirmative tú command would be:

• Pasa el semáforo = Go past the traffic light

But note that Pasa el semáforo en rojo would mean Go through the traffic light when it’s red, which is obviously not something you would normally tell someone to do.

The useful contrast is grammatical:

• Pasa = affirmative command
• No pases = negative command

This is a very important pattern in Spanish.

It is pronounced roughly seh-MA-fo-ro.

The stress is on má because of the written accent:

• se-MÁ-fo-ro

A rough English approximation is:

• seh-MAH-foh-roh

Also remember:

• the r in semáforo is a single tapped r, not a strong rolled rr

Yes, depending on context.

It can be understood as:

• Don’t drive through the red light
• Don’t cross when the light is red

However, if you want to be very specific:

For drivers:

• No pases el semáforo en rojo
• No te saltes el semáforo en rojo

For pedestrians:

• No cruces con el semáforo en rojo
• No cruces en rojo

So the original sentence is broad enough, but other versions can sound more precise depending on who is being addressed.