Learn Hardcore Portuguese (Portugal): Quem não perceber a explicação pode perguntar outra vez. - Whoever does not understand the explanation can ask again.

So it is not asking a direct question (like Quem não percebe? – Who doesn’t understand?), but introducing a general condition that applies to any person who fits it.

Why is it perceber and not percebe? What verb form is perceber here?

Here perceber is not an infinitive; it is the future subjunctive.

Portuguese has a future subjunctive that English does not. It is used in clauses referring to a possible future situation, often introduced by words like se, quando, quem, enquanto, logo que.

Quem não perceber a explicação…
Literally: Whoever will not understand / whoever does not come to understand the explanation… (future possibility)

For perceber, the future subjunctive forms are:

eu / ele / ela / você: perceber
nós: percebermos
vocês / eles / elas: perceberem

It looks the same as the infinitive for many verbs, but here you can see it’s a finite verb because it has a subject (quem) and heads its own clause.

Why is it the future subjunctive? Couldn’t it just be present tense, like Quem não percebe a explicação?

Quem não percebe a explicação is grammatically correct, but it sounds more like:

Whoever (generally) doesn’t understand the explanation – something habitual, true in general.

Using the future subjunctive (Quem não perceber a explicação) focuses on people in this future situation:

The teacher is probably talking about this specific explanation, right now, and what students may or may not understand as it goes on or afterwards.

So:

Quem não percebe a explicação – more general, habitual
Quem não perceber a explicação – future/conditional, about what may happen in this particular case

In classroom instructions, the future subjunctive is the natural choice.

Why is it pode perguntar (singular) and not podem perguntar (plural)? We’re talking about several people, aren’t we?

Grammatically, the verb normally agrees with quem, and quem is treated as singular:

Quem não perceber a explicação pode perguntar outra vez. (standard)

So pode is singular to match quem.

In real spoken Portuguese, you might occasionally hear plural agreement when it’s very clear we’re talking about a group, but the standard, correct form here is:

Quem … pode (singular)

If the teacher wants to make the plural very explicit, they usually rephrase:

Se não perceberem a explicação, podem perguntar outra vez.
(If you [plural] don’t understand the explanation, you can ask again.)

Does perceber always mean understand in European Portuguese? I thought it meant notice.

In European Portuguese (Portugal), perceber very commonly means understand:

Percebes o que estou a dizer? – Do you understand what I’m saying?
Não percebi a explicação. – I didn’t understand the explanation.

It can also mean realise / notice / figure out, depending on context:

Percebi que ele estava chateado. – I realised he was upset.

In Portugal, perceber is often the default everyday verb for understand, while entender is also used but can sound slightly more formal or careful in some contexts. In Brazil, learners will hear entender more often for understand, and perceber more for notice / realise.

So in this sentence, perceber = to understand.

What is the difference between outra vez, de novo, and mais uma vez?

All three can translate as again, but there are tendencies:

outra vez – very common in Portugal in everyday speech
- Pergunta outra vez. – Ask again.
de novo – literally from new, often also again, sometimes slightly more like afresh / over again
- Explica isso de novo. – Explain that again / once more.
mais uma vez – literally one more time, sometimes slightly more formal or emphatic
- Podem tentar mais uma vez. – You can try one more time.

In this sentence, perguntar outra vez is perfectly natural and neutral in European Portuguese: ask again.

Could we say perguntar de novo instead of perguntar outra vez? Would it change the meaning?

You can say perguntar de novo; the meaning is effectively the same here:

Quem não perceber a explicação pode perguntar de novo.

Both are understood as ask again.
In European Portuguese, outra vez is a bit more common in casual speech; de novo is also fine and fully idiomatic.

There is no real change in politeness or strength in this context; it’s just a stylistic preference.

Is a comma needed after explicação? For example: Quem não perceber a explicação, pode perguntar outra vez.

Both versions are seen, but standard punctuation does not require a comma here, and many style guides would prefer it without:

Quem não perceber a explicação pode perguntar outra vez. ✔ (preferred)

Portuguese allows a comma between a subordinate clause and the main clause, especially if there is a pause in speech, so:

Quem não perceber a explicação, pode perguntar outra vez. – also possible, especially in more relaxed writing.

However, in more formal or careful writing, you will often see it without the comma in sentences like this.

Could we change the word order and say Pode perguntar outra vez quem não perceber a explicação?

That is technically possible, but it sounds less natural and a bit heavier, especially as spoken classroom language.

Quem não perceber a explicação pode perguntar outra vez. – very natural, what a teacher would say.
Pode perguntar outra vez quem não perceber a explicação. – grammatically OK, but marked, more like a written or rhetorical style, and even then it may sound clumsy.

In general, for clear, natural Portuguese, keep the original order for this type of sentence.

What is the difference between perguntar and pedir? Could we say pode pedir outra vez here?

perguntar = to ask (a question)
pedir = to ask for / to request (something)

In this sentence, the idea is ask (a question) again, so perguntar is the correct verb:

pode perguntar outra vez – can ask again (i.e. ask the question again)

Pode pedir outra vez would mean can ask again for something (repeat a request, e.g. ask again for help, for a favour), which is not the usual way to say ask your question again.

So for asking questions, especially in class, you want perguntar.