Learn Hardcore Spanish (Latin America): Si llueve, entonces nos quedamos en casa.

It’s optional. The sentence is perfectly natural without it: Si llueve, nos quedamos en casa. Keeping entonces adds a clear “then/so” link: Si llueve, entonces nos quedamos en casa. In everyday speech, many speakers drop it.

Why is it llueve (present) and not lloverá (future) after si?

In Spanish, the si-clause takes the present indicative for future time. You do not use the future after si in this kind of condition. Common pairings:

Si llueve, nos quedamos en casa. (present–present)
Si llueve, nos quedaremos en casa. (present–future)
Si llueve, quédense en casa. (present–imperative)

Why not si llueva?

Because standard conditional si-clauses don’t use the present subjunctive. For unlikely/hypothetical conditions, Spanish uses the imperfect subjunctive:

Likely/real: Si llueve, nos quedamos/nos quedaremos.
Unlikely/hypothetical: Si lloviera/lloviese, nos quedaríamos.

Does nos quedamos mean “we stay” or “we’ll stay”?

Both are possible. In conditions like this, the present can refer to the future. Context decides:

General rule/plan: Si llueve, nos quedamos en casa.
Clear futurity/decision: Si llueve, nos quedaremos en casa.

Why do we need the reflexive nos in nos quedamos? Can I say quedamos en casa?

To mean “stay (remain) at home,” Spanish prefers the reflexive quedarse: nos quedamos en casa. Without the reflexive, quedar changes meaning:

Nos quedamos en casa. = We stay at home.
Quedamos en casa a las 8. = We agreed to meet at home at 8.
El cine queda cerca. = The theater is located nearby.

Why en casa and not a casa?

Use en for location and a for movement:

Location: Nos quedamos en casa.
Movement: Vamos a casa. / Regresamos a casa.

Do I need the article? en casa vs en la casa?

Both are possible, but they’re not identical in feel:

en casa = “at home” (neutral, general, very common).
en la casa = “in the house” (as a building) or a specific house; you might use it to contrast with being outside, or to refer to someone else’s house.

Can I change the word order? Where does the comma go?

Yes:

Si llueve, (entonces) nos quedamos en casa. Use a comma after the initial si-clause.
Nos quedamos en casa si llueve. No comma when the si-clause comes last.
Entonces normally sits at the start of the result clause: Si llueve, entonces nos quedamos...

What’s the difference between si llueve and cuando llueve?

Si llueve = if it rains (a condition that may or may not happen).
Cuando llueve = when(ever) it rains (habit or regular pattern). For a single future time that hasn’t happened yet, cuando takes the subjunctive: Cuando llueva, nos quedaremos en casa.

Is llueve different from está lloviendo here?

Yes:

Si llueve is general or neutral.
Si está lloviendo highlights the rain as ongoing at that moment: Si está lloviendo, nos quedamos en casa.

How do I express an unlikely or purely hypothetical situation?

Use the imperfect subjunctive + conditional:

Si lloviera/lloviese, nos quedaríamos en casa.

What’s the difference between si and sí?

si (no accent) = if: Si llueve...
sí (accent) = yes, or a reflexive pronoun meaning “himself/herself/themselves”: Sí, nos quedamos en casa. / Se cuida a sí mismo.

Can I say Si llueve, nos quedaríamos en casa?

It’s best to avoid that mix in standard grammar. The recommended patterns are:

Si llueve, nos quedamos/nos quedaremos.
Si lloviera, nos quedaríamos. You may hear the mixed form in parts of Latin America to soften the statement, but it’s not the textbook norm.

Is this phrasing the same in Latin America and Spain?

Yes. Si llueve, (entonces) nos quedamos en casa is understood everywhere. Minor differences are elsewhere (e.g., plural commands: quédense in most of Latin America vs quedaos in Spain), but the sentence itself is pan-Hispanic.