Learn Hardcore Italian: Sia Marco sia Lucia restano a casa perché piove forte. - Both Marco and Lucia are staying home because it’s raining hard.

Place sia immediately before each element: Sia Marco sia Lucia, sia a casa sia al lavoro, etc.
It’s symmetrical and doesn’t take a comma between the two parts.
It slightly emphasizes the inclusion of both items compared with a simple e (“and”).

Is this sia the same word as the subjunctive of essere?

They’re homographs but different functions:

As a conjunction: sia ... sia = “both ... and ...”.
As a verb: sia is the present subjunctive of essere (e.g., Spero che Marco sia a casa = “I hope Marco is at home”). In your sentence, it’s the conjunction, not the verb.

Why is the verb plural (restano) and not singular (resta)?

Because sia Marco sia Lucia forms a compound subject (two people). With compound subjects, Italian uses the third-person plural: restano. This is the same rule you’d use with Marco e Lucia.

Could I just say Marco e Lucia restano a casa? What’s the nuance?

Yes. Marco e Lucia restano a casa is the neutral, most common way. Sia Marco sia Lucia restano a casa adds mild emphasis to the fact that both of them are staying home.

Can I say sia Marco che Lucia instead of sia Marco sia Lucia?

Yes. Both are common:

sia A sia B (slightly more formal/neutral)
sia A che B (very common in speech) Alternatives with a similar meaning:
tanto A quanto B
non solo A, ma anche B All take a plural verb if they’re the subject.

What’s the difference between restare, rimanere, and stare here?

restare and rimanere are near-synonyms meaning “to stay/remain.” Either works: restano/rimangono a casa.
stare means “to be/stay” in a place: stanno a casa. It describes location/state. Using restare/rimanere can hint at the decision to not go out (remain at home), while stare simply states where they are. All three are acceptable; nuance is slight.

Why is it a casa and not alla casa or in casa?

a casa is the idiomatic way to say “at home/to home,” with no article: restare/andare/stare a casa.
in casa means “inside the house” (emphasis on being indoors): restano in casa is also fine.
alla casa would mean “to the house” (a specific building) and is not used for the idiomatic “home.”

Do I need a comma before perché?

Not necessarily. In short, clear sentences like this, omitting the comma is normal: ... restano a casa perché piove forte. A comma (... restano a casa, perché ...) is also acceptable stylistically, especially in longer sentences or to add a slight pause. Both are common in contemporary usage.

Does perché ever take the subjunctive?

With the meaning “because,” perché takes the indicative: perché piove forte.
With the meaning “so that/in order that,” it takes the subjunctive: Parlo piano perché tu capisca (= “so that you may understand”). A clearer alternative for purpose is affinché.

Why is there no subject with piove?

Weather verbs like piovere, nevicare, grandinare are impersonal. They appear in third-person singular with no explicit subject: piove, nevica, etc. You can also use periphrastic forms like sta piovendo (it’s raining).

Is piove forte correct? Should it be fortemente or molto?

Piove forte is natural and very common. In Italian, many adjectives work adverbially without -mente. Fortemente sounds odd with weather. Other idiomatic intensifiers:

piove molto / tanto
piove a dirotto / a catinelle
diluvia
piove fortissimo All are fine, with nuances of intensity/register.

Can I say sta piovendo instead of piove?

Yes:

Piove (forte) = a general statement about the rain now.
Sta piovendo (forte) = emphasizes the ongoing process (progressive aspect). Both are idiomatic; choose based on whether you want a more “right now” feeling.

Should I use the future tense if the rain is forecast?

If you refer to the future, yes: Domani resteranno a casa perché pioverà forte. That said, Italian often uses the present for near-future plans if the time is clear from context: Domani restano a casa perché piove forte (also acceptable in speech).

Can I move the perché clause to the beginning?

You can, but many speakers prefer siccome/poiché at the start:

Siccome/Poiché piove forte, sia Marco sia Lucia restano a casa. Starting with Perché is not wrong, but siccome/poiché avoids any fleeting “why?” ambiguity and sounds a touch more formal/natural sentence-initially.

How do I say “Neither Marco nor Lucia ...” in Italian?

Use né ... né ... with a plural verb:

Né Marco né Lucia escono, perché piove forte. If né ... né ... is not the subject, keep non in the clause:
Marco non esce né oggi né domani.

Why does perché have an accent, and which one is it?

It’s spelled with an acute accent on the final é: perché. The accent marks both stress and vowel quality. Perche without an accent is incorrect. Note that perché means both “because” and “why,” depending on context.