Baja del bordillo con cuidado, que viene una bicicleta.

Baja is the tú affirmative command of bajar.

• tú hablas → normal statement: you go down
• ¡Baja! → command: Get down! / Step down!

Compare:

• bajas = you go down / you are going down (statement)
• baja = go down! (command to tú)
• baje = command to usted (more formal)

So this sentence is speaking informally to one person.

Del is just the contraction of de + el.

• de el bordillo → del bordillo

Here, bajar de means to go down from / step down from something.

So:

• bajar del bordillo = to step down from the curb

This contraction is mandatory with de + el (except when el is part of a proper name).

In Spain, bordillo means the curb or kerb: the raised edge between the sidewalk and the street.

So:

• acera = sidewalk / pavement
• bordillo = curb / kerb
• calle or carretera = street / road

In this sentence, the person is being told to step down off the curb carefully.

Con cuidado is the most natural everyday way to say carefully in Spanish.

• Baja con cuidado = Step down carefully
• Cuidadosamente is grammatically correct, but it sounds more formal or less conversational in many everyday situations.

Spanish often prefers a phrase like con cuidado where English might use a single adverb.

Spanish usually leaves out subject pronouns when they are not needed.

The verb form already shows who the command is for:

• baja = command to tú
• baje = command to usted
• bajad = command to vosotros
• bajen = command to ustedes

So tú is unnecessary unless you want emphasis or contrast.

Yes, here que has an explanatory/causal meaning, especially in spoken Spanish. It can mean something like:

• because
• since
• watch out, because...

So:

• Baja del bordillo con cuidado, que viene una bicicleta. = Step down off the curb carefully, because a bicycle is coming.

This use of que is very common in natural speech, especially in Spain. It sounds more immediate and conversational than porque in this kind of warning.

Yes, you could. It would still be correct.

• ...que viene una bicicleta = very natural, spoken, immediate
• ...porque viene una bicicleta = also correct, a bit more explicitly because

In warnings and quick explanations, Spanish often prefers que.

Spanish often puts the verb before the subject when introducing new information.

So:

• viene una bicicleta = a bicycle is coming

This word order sounds very natural because the bicycle is being mentioned for the first time. It is similar to saying there’s a bike coming.

If you said una bicicleta viene, it would sound more marked or less natural in this context.

Because the bicycle is not something already identified.

• una bicicleta = a bicycle (some bicycle, not previously mentioned)
• la bicicleta = the bicycle (a specific one already known)

Here the speaker is warning about an approaching bike in general, so una is the normal choice.

In Spanish, the simple present often covers what English expresses with is coming.

So:

• viene una bicicleta = a bicycle is coming

Spanish does use estar + gerundio (está viniendo), but much less often than English uses is coming. In a quick warning, viene is the natural choice.

You might hear it, but baja del bordillo is more straightforward here.

• bajar = to go down / step down
• bajarse = to get off / get down oneself, often used with vehicles or places you are on/in

For example:

• Bájate del autobús = Get off the bus
• Bájate de la bicicleta = Get off the bicycle

With bordillo, plain baja sounds very natural. Bájate is not impossible, but it is less neutral here.

It sounds very natural in Spain, especially because of bordillo, which is a very Spain-Spanish word.

In other Spanish-speaking places, people might use different words for curb, such as:

• banqueta in some countries
• vereda in some places for sidewalk
• other regional terms depending on the country

Also, the overall rhythm of ..., que viene una bicicleta feels very typical of everyday Peninsular Spanish.

Yes. In everyday Spanish, especially in Spain, people very often say bici.

So you may hear:

• Cuidado, que viene una bici.

That sounds very natural and colloquial. Bicicleta is the full word, and it is perfectly correct here.

It sounds like a quick, natural warning or piece of advice.

It is:

• informal
• immediate
• practical
• very conversational

A parent, friend, or passerby might say it if someone is about to step off the curb while a bike is approaching. The que part makes it sound especially spontaneous and spoken.