Learn Hardcore Italian: Parla piano, per favore.

It’s the affirmative imperative of parlare, addressing tu (informal singular “you”). So Parla means “Speak!” directed at one person you’re on informal terms with.

Is this informal or formal? How would I say it formally?

It’s informal (to tu). The formal version (to Lei) is Parli piano, per favore. An even more polite request is Potrebbe parlare piano, per favore?

Why is it parla and not parli?

For -are verbs, the imperative with tu ends in -a: parla (speak), compra (buy). The form parli is either:

the formal imperative for Lei (e.g., Parli piano), or
the present subjunctive.

Does piano mean “slowly” or “quietly” in this sentence?

It can mean either “slowly” or “quietly/softly.” Context usually clarifies. To be unambiguous:

“more slowly”: più lentamente, più piano
“more quietly”: più piano, a bassa voce

Can I say Parla lentamente, per favore?

Yes. Lentamente explicitly means “slowly,” so Parla lentamente, per favore avoids the “quietly vs slowly” ambiguity.

How do I make the request sound less blunt?

Use a question or add softeners:

Informal: Per favore, puoi parlare un po’ più lentamente? / Scusa, potresti parlare più piano?
Formal: Per favore, potrebbe parlare più lentamente? / Scusi, potrebbe parlare a bassa voce?

Where can per favore go in the sentence?

It’s flexible. Common options:

Per favore, parla piano.
Parla piano, per favore. Both are standard. The comma is typical when per favore comes at the end.

How do I say it to more than one person?

Use the plural imperative voi: Parlate piano, per favore.

What’s the negative form?

For tu, the negative imperative uses the infinitive: Non parlare piano (“Don’t speak quietly/slowly”). For Lei (formal): Non parli piano. For voi (plural): Non parlate piano.

Where do pronouns go with the imperative?

Affirmative imperative: pronouns attach to the end: Parlami piano, per favore (“Speak to me softly, please”).
Negative imperative (tu): pronouns usually go after non and before the infinitive: Non mi parlare piano.

Why is there no subject pronoun (tu)?

Italian normally drops subject pronouns because the verb ending shows the subject. You could say Tu parla piano, but it adds emphasis and can sound scolding.

Does piano change form (gender/number)?

Not here. As an adverb meaning “slowly/softly,” piano is invariable. It doesn’t become piana/piani/piane.

What’s the nuance of più piano and piano piano?

Più piano = “more slowly/quietly.” Very common: Parla più piano, per favore.
Piano piano = “very slowly, gradually, step by step.” With speech it can mean “nice and slow/soft”: Me lo spieghi piano piano.