No bajes del bordillo hasta que el semáforo se ponga en verde.

Because Spanish uses the present subjunctive for negative tú commands.

• baja = affirmative informal command: step down
• no bajes = negative informal command: don't step down

So even though the sentence is a command, the negative form changes:

• Baja del bordillo. = Step off the curb.
• No bajes del bordillo. = Don’t step off the curb.

This is a very common pattern in Spanish:

• Hazlo / No lo hagas
• Ven / No vengas
• Cruza / No cruces

Yes. Bajar basically means to go down, to come down, or to lower. In this sentence, though, it is being used in a very natural everyday way:

• bajar del bordillo = to step down/off the curb

So the idea is not just a general downward movement, but specifically stepping off the curb into the road.

Bordillo means curb or kerb: the raised edge between the sidewalk and the street.

This is especially standard in Spain. A learner should know that vocabulary can vary by region, but in Spain bordillo is the normal word here.

It is different from:

• acera = sidewalk / pavement
• calzada = roadway
• bordillo = curb / kerb

So No bajes del bordillo is more specific than just saying don’t leave the sidewalk.

Because de + el contracts to del in Spanish.

So:

• de el bordillo → del bordillo

This contraction is normally mandatory.

The same happens with:

• a + el → al

Examples:

• bajar del coche = get out of the car / get down from the car
• ir al parque = go to the park

Because after hasta que, Spanish usually uses the subjunctive when the action is in the future or has not happened yet.

Here, the green light is something that is still pending, so Spanish says:

• hasta que el semáforo se ponga en verde

This is one of the standard triggers for the subjunctive.

Compare:

• Espera hasta que llegue. = Wait until he/she arrives.
• No salgas hasta que termine. = Don’t leave until it finishes.

But if you are talking about something completed in the past, you use the indicative:

• Esperé hasta que el semáforo se puso en verde. = I waited until the traffic light turned green.

Literally, it means becomes/turns green.

The verb here is ponerse, which often means to become or to turn in a change-of-state sense.

So:

• el semáforo se pone en verde = the traffic light turns green

This use of ponerse is very common:

• ponerse nervioso = to get nervous
• ponerse rojo = to turn red
• ponerse triste = to become sad

In this sentence, the traffic light is changing state.

It is part of the verb ponerse, which is often a pronominal verb. In many cases, it does not feel truly reflexive in English.

Here, ponerse means to become / to turn. So you should learn it as a unit:

• ponerse en verde
• ponerse rojo
• ponerse enfermo

So the se is not really it puts itself in a literal sense. It is just how Spanish builds this change-of-state meaning.

With traffic lights, en verde is a very natural expression in Spanish.

• estar en verde = to be green
• ponerse en verde = to turn green

This is especially common when talking about signals, lights, and similar systems.

So:

• el semáforo está en verde = the light is green
• el semáforo se pone en verde = the light turns green

Using en here sounds idiomatic and very natural.

Yes, that is also possible, and it is very natural.

There is a small nuance:

• se ponga en verde = until it turns green
• esté en verde = until it is green

So the original sentence focuses a little more on the change of color/state, while esté en verde focuses more on the resulting condition.

In practice, both are good Spanish.

Yes. No bajes is an informal singular command, used with tú.

So it is what you would say to:

• one child
• one friend
• one person you address with tú

Other versions would be:

• No baje del bordillo... = formal singular (usted)
• No bajéis del bordillo... = informal plural in Spain (vosotros)
• No bajen del bordillo... = formal plural or common plural in Latin America (ustedes)

In Spain, public safety messages often use tú to speak directly and clearly to the reader.

Not exactly, because they do not mean the same thing.

• acera = sidewalk / pavement
• bordillo = curb / kerb

So:

• No bajes del bordillo = Don’t step off the curb.
• No bajes de la acera = Don’t step off the sidewalk.

The second one is understandable, but the original is more precise because it focuses on the exact edge where you step into the street.

Yes. That would mean:

• Don’t cross until the light turns green.

The difference is in the action being forbidden:

• No bajes del bordillo... = Don’t even step off the curb yet.
• No cruces... = Don’t cross yet.

So the original sentence is slightly more specific and cautious: it tells you not to begin moving into the road at all.