Cuando el cielo está gris, prefiero quedarme en casa.

Cuando means when. It introduces a time clause: Cuando el cielo está gris = When the sky is gray.

In Spanish, this is a very common way to set the scene before the main idea. The main clause here is prefiero quedarme en casa.

Spanish uses estar for many states or conditions, and ser for more permanent or defining characteristics.

So:

• El cielo está gris = the sky is gray right now / at the moment
• El cielo es gris would sound more like gray is an essential characteristic of the sky, which is not what is meant here

Weather-related appearance and temporary conditions often use estar.

Gris is an adjective meaning gray.

In this sentence, it describes el cielo. A useful detail is that gris usually stays the same in masculine and feminine singular:

• el cielo gris
• la nube gris

In the plural, it often becomes grises:

• los cielos grises
• las nubes grises

So here, gris agrees with the singular noun cielo.

The verb is preferir (to prefer), and it is a stem-changing verb in the present tense.

The change is:

• e → ie in stressed forms

So:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

That is why the sentence uses prefiero: it means I prefer.

After preferir, Spanish often uses an infinitive to say what someone prefers to do:

• Prefiero salir = I prefer to go out
• Prefiero leer = I prefer to read
• Prefiero quedarme en casa = I prefer to stay at home

So prefiero is the main conjugated verb, and quedarme is the action being preferred.

Because the verb here is quedarse, not just quedar.

• quedar can mean things like to remain, to fit, to agree to meet, depending on context
• quedarse often means to stay or to remain behind

The -me is the reflexive pronoun, meaning myself in form, though in English we usually just say stay, not stay myself.

So:

• quedarme = to stay
• literally, something like to stay myself

In Spanish, reflexive pronouns can attach to an infinitive.

So you get:

• quedarme
• levantarme
• ducharme

With a conjugated verb + infinitive structure, Spanish often has two possibilities:

• Prefiero quedarme en casa
• Me prefiero quedar en casa ❌ this one is not correct here

For preferir + infinitive, the natural form is to attach the pronoun to the infinitive: quedarme.

En casa is the usual way to say at home or at the house/home in a general sense.

• Estoy en casa = I’m at home
• Prefiero quedarme en casa = I prefer to stay at home

Using en la casa is possible, but it usually sounds more specific, like in the house as a physical building, or in contrast with somewhere else.

So in this sentence, en casa is the natural expression.

The comma is natural and helpful because the sentence begins with a subordinate clause:

• Cuando el cielo está gris, prefiero quedarme en casa.

In Spanish, when a time clause comes first, a comma is commonly used before the main clause.

You may sometimes see sentences without the comma in informal writing, but using it here is good standard punctuation.

Not in this sentence as written.

Here, cuando refers to something habitual or real: whenever the sky is gray, I prefer to stay home. That takes the indicative:

• Cuando el cielo está gris...

The subjunctive after cuando is used when talking about a future or not-yet-real situation:

• Cuando el cielo esté gris, me quedaré en casa.
• When the sky is gray / when the sky turns gray, I’ll stay home.

So está is correct for a general present-time statement.

Yes, but the meaning shifts slightly.

• prefiero quedarme en casa = I prefer to stay at home
• prefiero estar en casa = I prefer to be at home

Quedarse emphasizes remaining there rather than going out.
Estar focuses more on the state/location itself.

In this context, quedarme en casa sounds very natural because it suggests choosing not to go out.

The sentence is completely natural in Spain.

Nothing in it is strongly region-specific, but it fits standard Peninsular Spanish very well:

• cuando
• el cielo está gris
• prefiero
• quedarme en casa

A speaker in Spain would say this exactly the same way. The only difference you might notice is pronunciation, such as the c/z sound in Spain, but the grammar and vocabulary are standard everywhere.

It has two parts:

1. Cuando el cielo está gris
   = a time clause introduced by cuando

2. prefiero quedarme en casa
   = the main clause

So the pattern is:

Cuando + clause, main clause

A similar example:

• Cuando llueve, leo en casa.
• When it rains, I read at home.

This is a very useful structure for talking about habits and preferences.