Learn Hardcore Portuguese (Portugal): Se não usares capacete, podes correr perigo na estrada. - If you don’t wear a helmet, you can be in danger on the road.

Se não usares capacete…
If you do not wear a helmet (in that future situation)…
Se não usas capacete…
If you don’t wear a helmet (in general / as a habit)…

So se não usares focuses on a future possibility, which fits the warning in the sentence.

Could I say Se não usas capacete, podes correr perigo na estrada? Would it be wrong?

It is grammatically correct, but the meaning shifts slightly.

Se não usares capacete…
Sounds like a warning about future situations:
If you ever do not wear a helmet, you may be in danger on the road.
Se não usas capacete…
Sounds more like a comment about a general habit:
If you don’t (usually) wear a helmet, you may be in danger on the road.

For the typical safety warning (thinking about what might happen in future rides), se não usares is more natural in European Portuguese.

Why is there no article before capacete? Why not o capacete or um capacete?

Portuguese can drop the article with a singular noun to talk about something in a general or generic way.

Se não usares capacete…
If you don’t wear helmet / if you do not use any helmet (helmets in general).

Compare:

Se não usares o capacete…
If you don’t wear the helmet (a specific helmet both speakers know about).
Se não usares um capacete…
If you don’t wear a helmet (at least one helmet, not a specific one).

In a general safety rule, não usares capacete (no article) is very natural, because we are talking about helmet use in general, not one particular helmet.

What exactly does correr perigo mean? Is correr really “to run” here?

Literally, correr perigo is to run danger, but in normal usage it means:

to be in danger
to be at risk

So:

podes correr perigo na estrada ≈ you may be in danger on the road.

It is a common fixed expression, similar to:

correr riscos – to run risks
estar em perigo – to be in danger

You could also say:

podes estar em perigo na estrada

That is correct, but correr perigo is a natural, idiomatic way to express the same idea.

Why do we say podes correr perigo instead of just corres perigo?

Poder + infinitive expresses possibility or permission.

podes correr perigo
you may be in danger / you can be in danger (it is possible)
corres perigo
you are in danger (it is definitely the case, not just possible)

In a warning like this, we want to say that danger is a possible consequence, not that it is guaranteed. So podes correr perigo is more appropriate than corres perigo.

Why is it na estrada and not em a estrada?

Portuguese usually contracts the preposition em with the definite article:

em + a = na
em + o = no
em + as = nas
em + os = nos

So:

em a estrada → na estrada (on the road)

Using the non‑contracted form em a estrada is considered incorrect in standard Portuguese in this context. The natural form is na estrada.

What is the difference between estrada and rua? Could I say na rua instead of na estrada?

Both estrada and rua can be translated as road in English, but they are used differently:

estrada
Road used mainly by vehicles, often between towns or outside built‑up areas; highways, country roads, etc.
rua
Street in a town or city, usually with houses, shops, pedestrians, etc.

So:

na estrada – on the road (as in traffic roads, highways, where you ride or drive)
na rua – on the street (in a town / city, often with pedestrians and buildings)

In the sentence about wearing a helmet, na estrada is more natural, because it suggests being out in traffic, on proper roads.

Is the comma after capacete necessary? How does punctuation work with se clauses?

When the if‑clause with se comes first, it is standard to use a comma:

Se não usares capacete, podes correr perigo na estrada.

If you reverse the order, the comma is usually omitted:

Podes correr perigo na estrada se não usares capacete.

So:

[If‑clause first] → comma before the main clause
[Main clause first] → normally no comma before the se clause

Why is there no tu in se não usares capacete? Where would it go if I wanted to say it?

Portuguese is a pro‑drop language: subject pronouns (like eu, tu, ele) are often left out because the verb ending already shows who the subject is.

(Tu) não usares – the ending ‑ares already tells us it is tu.

If you want to include tu for emphasis or clarity, it goes before the verb:

Se tu não usares capacete, podes correr perigo na estrada.

Both versions are correct. Without tu is more neutral and slightly more common in writing.

How would this sentence look in Brazilian Portuguese compared to European Portuguese?

In Brazil, people usually use você (third person) instead of tu (second person), and the future subjunctive form for você is usar, not usares. A natural Brazilian version would be:

Se você não usar capacete, pode correr perigo na estrada.

Main differences:

tu usares (European) → você usar (Brazilian)
podes (European tu) → pode (Brazilian você)

The structure and meaning are the same; only the pronoun and verb endings change.