Learn Hardcore Portuguese (Portugal): Se atravessares o cruzamento sem olhar, corres um grande risco. - If you cross the intersection without looking, you run a big risk.

In European Portuguese, after se (if) and certain time conjunctions (quando, logo que, assim que, enquanto), you normally use the future subjunctive when you’re talking about a future or hypothetical situation:

Se atravessares o cruzamento sem olhar…
If you (ever / in future) cross the intersection without looking…

It corresponds to English structures like:

If you cross…
When(ever) you cross…

English uses the present after if and when, but Portuguese often uses the future subjunctive in those same places when the event is future/uncertain.

Why is the first verb atravessares (future subjunctive) but the second verb corres (present indicative)? Why not use the same tense/mood in both parts?

This is a very typical pattern in Portuguese conditional sentences:

Se
- future subjunctive in the if-clause
Present indicative (or future) in the main clause

So:

Se atravessares o cruzamento sem olhar, corres um grande risco.

Literally: If you (should) cross the intersection without looking, you run a big risk.

Why present (corres) and not future (correrás)?

Corres presents the result as a general truth: any time this condition is met, this is the risk you run.
Correrás is not wrong, but it usually sounds more like a specific future event:
Se atravessares o cruzamento sem olhar, correrás um grande risco.
→ If you cross the intersection without looking (that time), you will run a big risk.

In everyday speech, the present in the main clause is more common in this kind of warning or general statement.

Could I say Se atravessas o cruzamento sem olhar, corres um grande risco instead? Is that wrong, or does it change the meaning?

It’s not strictly wrong, but it sounds different and is less natural for a neutral, general warning.

Se atravessares o cruzamento sem olhar…
→ The standard, neutral way to talk about a possible future situation (any time / ever in future).
Se atravessas o cruzamento sem olhar…
→ With the present indicative in the se-clause, this tends to sound more like:
- you’re talking about someone’s current, habitual behaviour, often with a tone of criticism or annoyance; or
- in some contexts, almost like “if it’s true that you cross…”, questioning or emphasising a fact.

Example of that nuance:

Se atravessas sempre o cruzamento sem olhar, um dia ainda vais ser atropelado.
If you (really) always cross the intersection without looking, one day you’ll get run over.

For a neutral, “if you cross (at any time / in general)” kind of rule or warning, future subjunctive (atravessares) is the default in European Portuguese.

Can I change the word order, like Corres um grande risco se atravessares o cruzamento sem olhar? Do I still need the comma?

Yes, you can change the order:

Se atravessares o cruzamento sem olhar, corres um grande risco.
Corres um grande risco se atravessares o cruzamento sem olhar.

Both are correct and mean the same.

About the comma:

When the if-clause comes first, a comma before the main clause is standard:
Se atravessares…, corres…
When the if-clause comes second, it is normally written without a comma in everyday usage:
Corres um grande risco se atravessares…

So the more natural punctuation is:

Se atravessares o cruzamento sem olhar, corres um grande risco.
Corres um grande risco se atravessares o cruzamento sem olhar.

What does sem olhar look like grammatically? Why is it the infinitive olhar and not something like olhares?

Sem olhar literally means “without looking”. Here:

sem = without
olhar = infinitive, to look

Portuguese often uses the infinitive after prepositions like sem, para, por, ao, antes de, depois de:

sem olhar – without looking
antes de comer – before eating
ao atravessar – when crossing / upon crossing

You could say sem olhares, which is the personal infinitive, 2nd person singular (tu):

Personal infinitive of olhar:
eu olhar, tu olhares, ele olhar, nós olharmos, vocês/eles olharem

So:

Se atravessares o cruzamento sem olhar…
→ neutral, generic: if you cross without looking (in general).
Se atravessares o cruzamento sem olhares…
→ grammatically correct, but more marked; it explicitly ties the action to you and often sounds more formal or literary in this context.

In everyday European Portuguese, sem olhar is by far the more natural choice here.

Why is it o cruzamento and not um cruzamento? Does the article change the meaning much?

Both are possible, but there is a nuance.

o cruzamento = the intersection
um cruzamento = an intersection

In Portuguese, the definite article (o / a / os / as) is used more often than in English, including:

For specific things:
- Para no cruzamento, à direita.
  Stop at the intersection, on the right.
For generic statements (more often than English):
- No cruzamento, tens de ter cuidado.
  At an intersection / At intersections, you have to be careful.

In your sentence:

Se atravessares o cruzamento sem olhar, corres um grande risco.

This can be understood as:

If you cross the intersection (e.g. the one we both see / the next one) without looking…
or more generically:
If you cross at the intersection without looking… (the “crossing situation”)

If you say:

Se atravessares um cruzamento sem olhar, corres um grande risco.

then you’re clearly talking about any intersection in general: If you cross an intersection without looking…

So:

o cruzamento → often feels more specific or more like a known/typical scenario, and is very natural in spoken EP.
um cruzamento → highlights that you mean any random intersection.

Why do you say corres um grande risco with the verb correr? Could I say something like tens um grande risco or estás em grande risco instead?

Portuguese has a strong idiom with correr + risco:

correr um risco = to run a risk
correr o risco de… = to run the risk of …ing

So:

corres um grande risco = you run a big risk

This is the natural, idiomatic collocation. Some related examples:

Estás a correr um risco enorme.
You’re running a huge risk.
Eles correram o risco de perder tudo.
They ran the risk of losing everything.

Alternatives:

estar em risco = to be at risk
→ often used for ongoing situations:
A tua vida está em risco. – Your life is at risk.
ter um risco (tens um risco) is not idiomatic here and usually sounds wrong in the sense of “you’re in danger”.

So in your sentence, corres um grande risco is exactly the natural expression.

Why is it um grande risco and not um risco grande? Does the position of grande change the meaning?

Adjectives in Portuguese can often go before or after the noun, but the position can affect nuance.

With grande:

um grande risco
→ feels more intensive / qualitative: a serious, considerable risk.
um risco grande
→ is possible, but it sounds more literal / descriptive (a big-sized risk), and is less idiomatic here.

Many adjectives that express subjective evaluation, intensity, or emotional colour tend to appear before the noun:

um grande amigo – a great friend
uma bela ideia – a lovely / great idea
um terrível erro – a terrible mistake

So um grande risco is the natural, idiomatic way to say “a big/serious risk”.
Um risco grande is not wrong, but sounds less polished and is used less in this abstract sense.

If I want to be formal or speak to more than one person, how does the sentence change? What about você vs tu?

Your original sentence uses tu (informal singular), so the verbs are in the 2nd person singular:

Se atravessares o cruzamento sem olhar, corres um grande risco.
(tu)

For different forms of “you” in European Portuguese:

Formal singular (você / o senhor / a senhora)
→ verbs in 3rd person singular:
- Se atravessar o cruzamento sem olhar, corre um grande risco.
Plural “you” (vocês)
→ verbs in 3rd person plural:
- Se atravessarem o cruzamento sem olhar, correm um grande risco.
Informal plural “you” (vós) – archaic / regional in Portugal, rarely used in everyday speech:
- Se atravessardes o cruzamento sem olhar, correis um grande risco.

In modern European Portuguese:

tu
- atravessares / corres → informal singular
você / o senhor / a senhora
- atravessar / corre → formal singular
vocês
- atravessarem / correm → plural (informal or neutral)

Would Brazilians say this sentence the same way, or is there a difference between European and Brazilian Portuguese here?

The grammar pattern (future subjunctive after se) is the same in Brazilian Portuguese, but:

Brazilians mostly use você instead of tu.
The spoken style is a bit different.

Likely Brazilian versions:

Se você atravessar o cruzamento sem olhar, corre um grande risco.
Se você atravessar a rua sem olhar, corre um grande risco.

Points to notice:

Future subjunctive of atravessar with você is atravessar (same form as the infinitive), so it looks like the infinitive but is grammatically future subjunctive.
Você takes 3rd person verbs:
- você atravessar, você corre
Vocabulary differences: Brazilians might more often say a rua (the street) instead of o cruzamento (the intersection), depending on context, though cruzamento also exists.

So structurally it’s parallel, but:

EP informal: Se atravessares o cruzamento sem olhar, corres um grande risco.
BP neutral: Se você atravessar o cruzamento sem olhar, corre um grande risco.

Could I replace se with quando here? What’s the difference between se atravessares and quando atravessares?

You can, but it changes the meaning slightly:

Se atravessares o cruzamento sem olhar, corres um grande risco.
→ If you cross the intersection without looking, you run a big risk.
se = if → it’s conditional / uncertain. Maybe you will, maybe you won’t.
Quando atravessares o cruzamento sem olhar, corres um grande risco.
→ When you cross the intersection without looking, you run a big risk.
quando = when → it assumes that this will happen (at some point).

Both use the future subjunctive (atravessares) because they refer to a future event, but:

se introduces a condition.
quando introduces a future time which is taken as given/certain.

In most warning/“don’t do this” contexts, se is more natural, because you are talking about the risk if the person does that action.