Learn Hardcore Portuguese (Brazil): Se eu cometer um erro na prova, vou ficar nervoso. - If I make a mistake on the test, I will get nervous.

cometer is used with certain “bad” or undesirable actions:
- cometer um erro – to make a mistake
- cometer um crime – to commit a crime
- cometer uma infração – to commit an infraction

Saying fazer um erro is understood, but it sounds foreign or incorrect to native speakers. You can say fazer uma besteira (“to do something stupid”) or fazer uma cagada (very informal), but with erro the natural verb is cometer.

What exactly is going on grammatically in se eu cometer? What tense or mood is cometer here?

In se eu cometer um erro, the verb cometer is in the future subjunctive (futuro do subjuntivo).

The future subjunctive is used in clauses introduced by words like se, quando, assim que, logo que when they refer to a possible future event.
It expresses something that might happen in the future, not something that is happening now or that always happens.

Pattern:

Se eu cometer um erro… – If I make a mistake (in the future)…
Quando eu chegar em casa… – When I get home…
Assim que ele terminar, me liga. – As soon as he finishes, call me.

So cometer here is not an infinitive; it’s specifically the future subjunctive form of cometer.

Why can’t we say se eu cometo um erro instead of se eu cometer um erro?

You can say se eu cometo um erro, but it means something different.

Se eu cometer um erro, vou ficar nervoso.
- Conditional about the future.
- Roughly: If I (happen to) make a mistake (on that test), I will get nervous.
- Focus: a specific future situation (that exam).
Se eu cometo um erro, fico nervoso.
- General truth / habit (present indicative).
- Roughly: If I make a mistake, I get nervous.
- Focus: what usually happens, a pattern of behavior.

In your sentence, we’re talking about a specific future exam, so Portuguese normally uses se + future subjunctive: se eu cometer.

Why is it vou ficar nervoso instead of the simple future ficarei nervoso?

Brazilian Portuguese strongly prefers the “ir” + infinitive future in speech:

vou ficar nervoso = I’m going to get nervous / I will get nervous

The simple future ficarei exists, but:

it sounds formal, written, or literary,
it’s rare in everyday spoken Brazilian Portuguese,
most Brazilians practically never say ficarei in normal conversation.

So:

Natural spoken: Se eu cometer um erro na prova, vou ficar nervoso.
More formal / written: Se eu cometer um erro na prova, ficarei nervoso.

Both are grammatically correct; the first is what you’ll actually hear.

What does ficar nervoso mean exactly? How is it different from estar nervoso or ser nervoso?

The verb ficar here means “to become / to get”.

ficar nervoso = to become nervous / to get nervous (a change of state)
vou ficar nervoso = I’m going to get nervous

Compare:

estar nervoso – to be nervous (temporary state, right now)
- Eu estou nervoso. – I’m nervous (at the moment).
ser nervoso – to be a nervous person (character trait)
- Eu sou nervoso. – I’m a nervous/anxious type.
ficar nervoso – to become nervous in a given situation
- Eu sempre fico nervoso em provas. – I always get nervous in exams.

In your sentence, we’re talking about a reaction caused by a possible mistake, so ficar (change of state) is the right verb.

Why is it na prova and not no prova or em a prova?

Prova is a feminine noun, so:

a prova = the test / the exam

In Portuguese, em + a contracts to na:

em
- a prova → na prova = in / on the test

Because prova is feminine, you say:

na prova (correct)
no prova (wrong, that would imply a masculine noun)
em a prova (grammatically wrong; you must use the contraction).

If the noun were masculine, you’d get no:

no exame = em + o exame

Does prova always mean “test/exam”? Can it mean “proof” too?

Prova is a very common word with several meanings. Two important ones for you:

Test / exam (most common in school/university contexts)
- Tenho uma prova amanhã. – I have a test tomorrow.
- That’s the meaning in your sentence.
Proof / evidence
- Você tem alguma prova disso? – Do you have any proof of that?

The meaning is decided by context.
In a school context, prova almost always = test or exam.

Why is it um erro instead of just erro with no article?

In Portuguese, it’s very natural to use the indefinite article (um / uma) in cases where English might drop a or use no article.

cometer um erro – to make a mistake (one mistake, some mistake)

If you said se eu cometer erro na prova, it would sound incomplete or unnatural. The indefinite article here:

makes the phrase sound natural and idiomatic,
stresses “any one mistake” rather than the idea of error in general.

You can omit the article in certain fixed expressions with erro (e.g., sem erro = no mistakes), but not in cometer erro in this context.

Can I change the word order and say Vou ficar nervoso se eu cometer um erro na prova?

Yes. Both orders are correct:

Se eu cometer um erro na prova, vou ficar nervoso.
Vou ficar nervoso se eu cometer um erro na prova.

The meaning is the same.

Starting with se-clause is very common and slightly emphasizes the condition.
Starting with vou ficar nervoso focuses more on the result first, then the condition.

In everyday speech, both patterns are used freely.

Why is it nervoso and not something else? What if the speaker is a woman?

Nervoso is an adjective that agrees in gender with the subject.

For a male speaker:
- vou ficar nervoso – I (male) will get nervous
For a female speaker:
- vou ficar nervosa – I (female) will get nervous

So the sentence as given assumes the speaker is grammatically masculine. If a woman is talking, she would naturally say:

Se eu cometer um erro na prova, vou ficar nervosa.

The verb ficar doesn’t change; only the adjective nervoso/nervosa changes.

How do you pronounce prova, erro, and nervoso? Anything tricky for English speakers?

Approximate Brazilian Portuguese pronunciation (Rio/São Paulo–type accent):

prova → [PROH-vah]
- pr as in English “pro”,
- o like in British “hot”,
- final a like the “a” in “father”, but shorter.
erro → [EH-hoo] (or [EH-hu])
- first e like “bet”,
- rr is a voiceless h-like sound in most of Brazil,
- final o is a short “oo”/“u”-ish sound in many accents.
nervoso → [nehr-VOH-zoo] (or [neʀ-VOH-zu])
- ner like “nehr”,
- stressed vo like “vo” in “vote” but without a diphthong,
- so often sounds like “zu”/“zooh” in Brazilian Portuguese.

Nothing extremely difficult, but English speakers often:

over-roll the r in erro instead of using the Brazilian h-like sound,
make the final o too strong like English “oh” instead of a shorter Brazilian vowel.

Could I say se eu errar na prova instead of se eu cometer um erro na prova? Does it mean the same thing?

Yes, se eu errar na prova is very natural and means almost the same thing.

cometer um erro = to commit an error (a bit more formal / explicit)
errar = to make a mistake, to be wrong

Compare:

Se eu cometer um erro na prova, vou ficar nervoso.
Se eu errar na prova, vou ficar nervoso.

Both are correct and idiomatic. Errar is shorter and more common in everyday speech. Cometer um erro can sound a bit more careful or emphatic.

Are there any small changes that would still be correct and sound natural, like adding eu or algum?

Yes, several small variations are common and natural in Brazilian Portuguese:

Adding eu before vou (more explicit subject):
- Se eu cometer um erro na prova, eu vou ficar nervoso.
Using algum erro (“any mistake / some mistake”) for emphasis on “any”:
- Se eu cometer algum erro na prova, vou ficar nervoso.
Moving na prova:
- Se eu cometer um erro, vou ficar nervoso na prova.
  (Here it sounds more like I’ll be nervous during the test rather than nervous because of the mistake.)

All of these are grammatically correct; the original is already perfectly natural.