Como hoy hace frío, prefiero quedarme en casa.

In this sentence, como is being used as a conjunction meaning since, as, or because.

• Como hoy hace frío, ... = Since it's cold today, ...

This use is especially common when the reason comes first in the sentence. If como appears at the beginning like this, it often introduces the cause.

Compare:

• Como hoy hace frío, prefiero quedarme en casa.
  = Since it's cold today, I prefer to stay at home.
• ¿Cómo estás?
  = How are you?

So the meaning depends on context and position in the sentence.

Yes, but the sentence structure would usually change.

Most natural options:

• Como hoy hace frío, prefiero quedarme en casa.
• Prefiero quedarme en casa porque hoy hace frío.

Both are correct, but they are used slightly differently:

• como often introduces the reason first
• porque usually gives the reason after the main statement

So for this sentence, como sounds very natural because the speaker starts with the reason.

Because the sentence begins with a subordinate clause:

• Como hoy hace frío, = dependent clause
• prefiero quedarme en casa. = main clause

In Spanish, when this kind of explanatory clause comes first, a comma is normally used before the main clause.

So the comma helps separate:

• the reason
• the main action or preference

In Spanish, weather expressions often use hacer.

So Spanish says:

• hace frío = it is cold
• hace calor = it is hot
• hace buen tiempo = the weather is nice

This is just a standard weather pattern. English uses it is, but Spanish often uses hacer.

Why not the others?

• es frío usually describes something as cold by nature or character
• está frío usually means something is cold at the moment, like food, a room, or your hands

But for general weather, Spanish normally uses hace frío.

Because Spanish often leaves out subject pronouns when the verb ending already makes the subject clear.

• prefiero already means I prefer

The ending -o tells you it is yo.

So:

• (Yo) prefiero quedarme en casa.

Both are possible, but yo is usually omitted unless you want emphasis, contrast, or clarity.

For example:

• Yo prefiero quedarme en casa, pero ella quiere salir.

Here yo is useful because you are contrasting I with she.

Because preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• prefiero
• prefieres
• prefiere
• preferimos
• preferís
• prefieren

So:

• yo prefiero
• not yo prefero

This is a very common kind of verb change in Spanish.

Because quedarse means to stay or to remain, while quedar on its own can mean several different things, such as:

• to remain
• to be left
• to arrange to meet
• to suit / fit

In this sentence, the intended meaning is to stay at home, and Spanish normally uses the reflexive form:

• quedarse en casa

So:

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

Using quedar without me here would sound unnatural for this meaning.

Because when a reflexive verb comes after a conjugated verb and stays in the infinitive, the reflexive pronoun can attach directly to the infinitive.

Here:

• prefiero = conjugated verb
• quedarme = infinitive + reflexive pronoun

This is the normal structure:

• prefiero quedarme
• quiero acostarme
• voy a sentarme

With infinitives, attaching the pronoun is standard.

You cannot say:

• prefiero me quedar ❌

The pronoun must go in the correct place, and with an infinitive it is usually attached to the end.

En casa is the normal expression for at home.

• en casa = at home

Why not the others?

• a casa is not standard in Spanish in this context
• en la casa usually means in the house, referring more to the physical building

So there is a difference:

• Estoy en casa. = I’m at home.
• Estoy en la casa. = I’m in the house.

The second one is possible, but it sounds more literal and less idiomatic for the general idea of being at home.

Hoy is the time expression: today.

Its placement is flexible, but in this sentence it naturally modifies the weather expression:

• Como hoy hace frío... = Since today it’s cold...

Other placements are also possible in Spanish, depending on emphasis:

• Como hace frío hoy, prefiero quedarme en casa.
• Hoy, como hace frío, prefiero quedarme en casa.

The chosen version sounds very natural and smooth.

Yes, but it is slightly different.

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

Quedarse emphasizes the idea of remaining somewhere instead of going out or leaving.
That makes it especially suitable here.

So if the idea is I’d rather stay in, quedarme en casa is the better choice.

Yes, it sounds completely natural in Spain and also in many other Spanish-speaking places.

Nothing in the sentence is unusually formal or regional. It is just a very standard, everyday Spanish sentence.

A speaker from Spain would find all of these perfectly normal:

• Como hoy hace frío, prefiero quedarme en casa.
• Hoy hace frío, así que prefiero quedarme en casa.
• Prefiero quedarme en casa porque hoy hace frío.

So this is a very useful model sentence for everyday conversation.