Senhor, faça marcha-atrás devagar e não comece a buzinar logo.

Because Senhor is being used as a direct form of address — basically sir. In Portuguese, as in English, a vocative is separated by a comma:

Senhor, faça marcha-atrás...

So the comma is not optional here. It shows that the speaker is addressing someone directly.

Here it means sir.

It is not being used like Mr Smith. It is being used on its own to address a man politely or formally. In the middle of a sentence it would usually be written senhor with a lowercase s; it is capitalized here because it is the first word of the sentence.

Faça is the command form used for a formal/polite you in Portuguese.

The verb is fazer (to do / to make), and in this sentence the implied subject is something like o senhor. In European Portuguese, commands to o senhor use the same form as the present subjunctive:

• faça = do / make (formal command)

If you were speaking informally to tu, you would say:

• Faz marcha-atrás devagar.

So faça signals politeness or distance.

Yes. Portuguese often omits the subject when it is clear from the verb form or context.

Here the implied subject is o senhor:

• (O senhor) faça marcha-atrás devagar...

The speaker does not need to say it explicitly because Senhor already shows who is being addressed.

Marcha-atrás literally refers to reverse gear or reversing.

In the expression:

• fazer marcha-atrás

it means to reverse / back up a vehicle.

So this is a very natural driving-related phrase in Portuguese. It is not literally make backwards march in the way English would phrase it; it is just the normal Portuguese expression.

Because it is treated as a fixed compound expression.

In Portuguese, some compound nouns are written with a hyphen, and marcha-atrás is one of them. In this sentence it functions as a unit, not as two separate words with fully independent meaning.

So:

• marcha-atrás = reverse / reversing / reverse gear

Because devagar is the more natural everyday word here.

Both can mean slowly, but:

• devagar = common, everyday, very natural in speech
• lentamente = more formal, more bookish, sometimes more deliberate in tone

In instructions like this, devagar is exactly what you would expect to hear.

This is a negative command in the formal/polite form.

With o senhor (or você in grammar terms), Portuguese uses the present subjunctive for commands, especially in negative commands:

• comece = start (formal command)
• não comece = don’t start (formal negative command)

So the structure is perfectly regular for formal speech.

If it were informal tu, it would be:

• não comeces

Because começar a + infinitive means to start doing something.

So:

• não comece a buzinar = don’t start honking
• não buzine = don’t honk

The version with comece a adds a nuance like:

• don’t go starting to honk
• don’t immediately begin honking

It suggests annoyance at the idea of someone starting that action straight away.

Yes. Buzinar is a normal verb meaning to honk or to sound the horn.

It comes from buzina, which means horn. So:

• buzinar = to honk
• a buzinar = honking / to honk

It is a perfectly ordinary driving-related verb.

Here logo means right away, immediately, or straight off.

So:

• não comece a buzinar logo

means something like:

• don’t start honking immediately
• don’t start honking straight away

This is a very common use of logo in Portuguese. It does not mean an English logo like a brand symbol.

It is formally worded, but not especially warm.

It sounds polite in grammar because of:

• Senhor
• faça
• não comece

But the overall tone may still sound a bit irritated or firm, because the speaker is giving instructions and telling the person not to honk immediately.

To make it softer, you could add por favor:

• Senhor, faça marcha-atrás devagar e não comece logo a buzinar, por favor.

So the grammar is polite, but the message itself may still feel impatient.