Learn Hardcore Portuguese (Brazil): Por favor, use a faca para cortar o bolo.

Use is the affirmative command (imperative) for você (and also o senhor / a senhora). It comes from the present subjunctive form of usar: que você use → command: Use!
If you’re speaking to tu (common in some regions, less in much of Brazil), the command is typically usa: Usa uma faca…
Usar is the infinitive (to use) and wouldn’t be used alone as a command here.

Who is being addressed—você or tu—since neither appears in the sentence?

In Brazilian Portuguese, commands often omit the subject pronoun. The form use strongly implies você (formal/neutral).
If it were aimed at tu, you’d commonly see usa (or in some regions use can still appear with tu, but that’s more of a mismatch between pronoun and verb form).

Is Por favor always used at the beginning? Can it go elsewhere?

No. Por favor is flexible:

Por favor, use uma faca para cortar o bolo.
Use uma faca, por favor, para cortar o bolo.
Use uma faca para cortar o bolo, por favor.
Beginning position is very common and sounds natural.

Do I need the comma after Por favor?

It’s optional but common in writing. With Por favor as a parenthetical politeness marker, a comma helps readability: Por favor, use…
In casual messages, people often omit it: Por favor use…

Why is it uma faca (or a faca)—what’s the difference?

Your sentence has a faca (the knife), which can sound like a specific knife is assumed/available.
Uma faca (a knife) is often more natural if you just mean any knife: Por favor, use uma faca…
Both are correct; choice depends on context:

Specific/obvious knife: a faca
Any suitable knife: uma faca

Why is it para cortar and not para cortando?

After para to express purpose (in order to), Portuguese uses para + infinitive: para cortar.
Cortando is the gerund (cutting) and wouldn’t follow para in this structure.

Could I say Pra instead of Para?

Yes in speech and informal writing: Por favor, use uma faca pra cortar o bolo.
Para is more formal and preferred in careful writing.

Why is it o bolo and not just bolo?

Portuguese commonly uses articles (o/a/os/as) where English might omit the.
o bolo means the cake (a specific cake in context). You can drop the article in some contexts (more generic/recipe-like), but cortar o bolo is the normal everyday phrasing.

Is cortar the best verb here? What about partir or fatiar?

cortar o bolo is the standard, general way to say cut the cake.
Other options change the nuance:

fatiar o bolo = slice the cake (into slices)
partir o bolo = split/break the cake (less typical for neatly serving cake) So cortar is the safest default.

Could this be phrased with com instead of use?

Yes. Two natural alternatives are:

Por favor, corte o bolo com uma faca. (Please cut the cake with a knife.)
Por favor, corte o bolo usando uma faca. (…using a knife.)
Your original sentence (Use a knife to cut the cake) is also perfectly natural; it just foregrounds the tool first.

Is use here a request or more like an instruction?

It’s an imperative, so grammatically it’s a command, but Por favor makes it polite and commonly understood as a request. Tone and context decide how strong it feels.

How do I pronounce key parts like use, faca, and bolo in Brazilian Portuguese?

Approximate (Brazilian) pronunciation:

use: OO-zee (the s sounds like z between vowels)
faca: FAH-kah (first syllable stressed)
bolo: BOH-loo (final o often sounds like oo in many accents)

Can I add a pronoun like me or pra mim to make it “Use a knife for me”?

Yes, but it changes the meaning:

Por favor, use uma faca para cortar o bolo. (neutral request/instruction)
Por favor, corte o bolo pra mim. (Please cut the cake for me.)
If you want “for me,” Portuguese usually attaches it to the action (cortar) rather than to use.