Learn Hardcore Portuguese (Portugal): Quando não sei uma palavra, peço ao professor que me empreste um dicionário. - When I don’t know a word, I ask the teacher to lend me a dictionary.

saber = to know information, facts, how to do something.
- não sei uma palavra = I don’t know a particular word (I don’t have that information in my memory).
conhecer = to be familiar with a person, place, or thing in a more general sense.
- conheço essa palavra would sound more like “I’ve come across that word / I’m familiar with that word.”

In this context (vocabulary you don’t know), saber is the natural verb: Quando não sei uma palavra…

Why is there a comma after palavra?

The comma separates two clauses:

Quando não sei uma palavra – a dependent clause introduced by quando (when).
peço ao professor que me empreste um dicionário – the main clause (what I do in that situation).

In Portuguese, a dependent clause that comes before the main clause is normally followed by a comma. If the order were reversed, you’d usually drop the comma:

Peço ao professor que me empreste um dicionário quando não sei uma palavra. (no comma needed)

Why is peço used and not pergunto?

Both verbs can translate as “ask,” but they’re used differently:

perguntar = to ask a question, request information
- Pergunto ao professor o significado da palavra. = I ask the teacher the meaning of the word.
pedir = to ask for something or to request that someone do something
- Peço um dicionário ao professor. = I ask the teacher for a dictionary.
- Peço ao professor que me empreste um dicionário. = I ask the teacher to lend me a dictionary.

Here you’re asking the teacher to do something (lend you a dictionary), so pedir (peço) is correct.

What does ao professor mean exactly, and why not just a o professor?

ao is a contraction:

a (to) + o (the, masculine singular) → ao

So ao professor literally means “to the teacher.”

Portuguese almost always contracts a + o(s)/a(s):

a + o livro → ao livro (to the book)
a + os alunos → aos alunos (to the students)
a + a professora → à professora (to the female teacher)
a + as professoras → às professoras (to the female teachers)

You would not write a o professor; the contracted form ao is required.

Could we say para o professor instead of ao professor?

In this sentence, ao professor is the normal and most natural choice, because the teacher is the indirect object (the person you ask).

pedir algo a alguém = to ask someone for something
- Peço um dicionário ao professor.

para o professor usually gives more of a destination or purpose idea (“for the teacher”, “to be given to the teacher”), not “I am addressing him with a request.” It would sound off here.

So: peço ao professor (I ask the teacher), not peço para o professor.

What is the role of que in peço ao professor que me empreste um dicionário?

Here que is a conjunction meaning that, introducing a subordinate clause that expresses what you are asking the teacher to do:

peço ao professor – I ask the teacher
que me empreste um dicionário – that he lend me a dictionary

It’s very similar to English:

“I ask the teacher that he lend me a dictionary”
(in normal English you’d say “to lend me”, but grammatically that
- clause matches the Portuguese structure).

Why is it empreste and not empresta?

empreste is the present subjunctive of emprestar.

After verbs like pedir, querer, mandar, dizer (to tell), when they express a request, wish, order, or recommendation, Portuguese generally uses:

verb + que + [subjunctive]

So:

Eu peço que ele empreste um dicionário.
→ empreste (subjunctive), not empresta (indicative).

In your sentence:

peço ao professor que me empreste um dicionário
= I ask the teacher to lend me a dictionary (literally: I ask that he lend me a dictionary).

Using empresta here would sound incorrect to a native speaker.

Why is the pronoun me before the verb (me empreste) and not after it (empreste-me)?

In European Portuguese, the position of unstressed object pronouns (me, te, se, lhe, nos, vos, lhes) follows specific rules.

In a clause introduced by que, you normally get proclisis (pronoun before the verb):

que me empreste – standard EP order here.

Empreste-me is possible in main clauses in some contexts (especially written/formal style), but after que in this kind of sentence, me empreste is the natural and expected form.

So:

✅ que me empreste um dicionário
❌ que empreste-me um dicionário (ungrammatical in EP in this context)

Could I say peço ao professor um dicionário instead of peço ao professor que me empreste um dicionário?

Yes, that’s possible, and it’s simpler:

Peço ao professor um dicionário. = I ask the teacher for a dictionary.

The difference is:

peço ao professor um dicionário focuses on the thing requested (a dictionary).
peço ao professor que me empreste um dicionário focuses more on the action you want him to perform (to lend you one).

Both are correct and natural; your original sentence is just a bit more explicit about the lending action.

Why is it um dicionário and not o dicionário?

um dicionário = a dictionary, any dictionary, not a specific one you have in mind.
o dicionário = the dictionary, a specific dictionary that both speaker and listener can identify (for example, the one on the teacher’s desk).

In this sentence, you mean “a dictionary in general, any dictionary the teacher can lend,” so um dicionário is the normal choice.

How would the sentence change if the teacher is a woman?

You only need to change professor (male teacher) to professora (female teacher):

Quando não sei uma palavra, peço à professora que me empreste um dicionário.

Notice that:

a + a professora → à professora (with grave accent)
So the masculine ao professor becomes feminine à professora.

Why is it Quando não sei and not Quando não souber?

Both are possible, but they express different time meanings.

Quando não sei uma palavra, peço ao professor…
- Present indicative sei / peço
- Describes a general, habitual action: Whenever I don’t know a word, I (usually) ask the teacher…
Quando não souber uma palavra, vou pedir ao professor…
- souber is future subjunctive
- Describes a specific or future situation: When I don’t know a word (in the future, in some situation), I’ll ask the teacher…

In your sentence, the idea is a habit or routine, so the present indicative sei fits best.

What’s the difference between emprestar and pedir emprestado?

emprestar = to lend (the subject is the person who gives the thing):
- O professor empresta-me um dicionário.
  The teacher lends me a dictionary.
pedir emprestado = to ask to borrow (the subject is the person who wants to receive the thing):
- Peço (ao professor) um dicionário emprestado.
  I ask (the teacher) to borrow a dictionary.

Your sentence is structured as:

peço ao professor que me empreste um dicionário
= I ask the teacher to lend me a dictionary
(which is equivalent in meaning to “I ask to borrow a dictionary from the teacher”).