No aceleres si el coche de delante frena.

Because No aceleres is a negative tú command: Don’t accelerate.

In Spanish, negative commands for tú use the present subjunctive form:

• acelera = accelerate! / you accelerate
• no aceleres = don’t accelerate

So:

• Acelera = positive command
• No aceleres = negative command

This is a very common pattern:

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

Because after si meaning if, Spanish normally uses the indicative when talking about a real, possible condition.

So:

• si el coche de delante frena = if the car in front brakes

Here, frena is the present indicative of frenar.

English learners sometimes expect a subjunctive because the sentence is about a possibility, but Spanish does not use the subjunctive after si in this kind of normal present/future condition.

Compare:

• Si llueve, no salgo. = If it rains, I’m not going out.
• Si viene, avísame. = If he comes, let me know.

El coche de delante means the car in front or the car ahead.

Literally, it is something like the car from/in front, but in natural English you would translate it as:

• the car in front
• the car ahead

In Spanish, de delante is a common way to describe the one in front:

• la casa de delante = the house in front
• el coche de delante = the car in front

Because delante by itself is usually an adverb meaning in front or ahead, while de delante works like the one in front when attached to a noun.

Compare:

• El coche está delante. = The car is in front.
• El coche de delante frena. = The car in front is braking.

So in your sentence, de delante identifies which car we mean.

This is a very common question.

• delante usually refers to position: in front
• adelante often suggests forward, ahead, or go ahead

Examples:

• Está delante de mí. = He/She is in front of me.
• Sigue adelante. = Keep going forward.
• Adelante. = Come in / Go ahead.

In el coche de delante, the idea is positional, so delante is the natural choice.

Because si without an accent means if.

Spanish distinguishes:

• si = if
• sí = yes

Examples:

• Si llueve, no voy. = If it rains, I’m not going.
• Sí, claro. = Yes, of course.

So in your sentence, si is the conditional word if, which never takes an accent.

Not exactly. Frenar means to brake or to slow down by braking.

So el coche de delante frena means:

• the car in front brakes
• the car in front is braking

It does not necessarily mean the car comes to a complete stop.

That is why this sentence is good driving advice: if the car ahead brakes, don’t accelerate.

Because Spanish often omits subject pronouns when they are clear from the verb form or the context.

Here:

• aceleres clearly refers to tú
• frena clearly refers to el coche de delante

So Spanish does not need to say:

• No tú aceleres...
• ...si él frena

Those would sound unnecessary here.

This is normal in Spanish:

• Como = I eat / I’m eating
• Vienes = you come / you’re coming
• Frena = he/she/it brakes

It is informal singular, because no aceleres is the negative command for tú.

If you wanted other versions, they would be:

• No aceleres = don’t accelerate (informal singular, Spain and much of the Spanish-speaking world)
• No acelere = don’t accelerate (formal singular, usted)
• No aceleréis = don’t accelerate (informal plural, vosotros, used in Spain)
• No aceleren = don’t accelerate (formal plural or general plural, ustedes)

So the original sentence sounds like a normal instruction to one person, in an informal register.

Coche is the standard word for car in Spain, so it fits the Spanish (Spain) context very well.

Other Spanish-speaking countries often prefer:

• carro
• auto
• automóvil

A learner of Spain Spanish should treat coche as the most natural everyday word here.

You could say cuando, but it would change the meaning slightly.

• No aceleres si el coche de delante frena. = Don’t accelerate if the car in front brakes.
• No aceleres cuando el coche de delante frena. = Don’t accelerate when the car in front brakes.

Si makes it a condition.
Cuando makes it sound more like a situation that happens at that moment or whenever it happens.

In driving instructions, si is usually the more precise choice for if.

Yes. In a driving context, acelerar can mean both:

• to accelerate
• to speed up

So No aceleres can naturally be understood as:

• Don’t accelerate
• Don’t speed up

The exact English wording depends on the context, but both ideas are very close here.