Si vamos a la sierra en invierno, más vale llevar ropa de abrigo.

Because after si for a real, possible condition, Spanish normally uses the present indicative, not the subjunctive.

So:

• Si vamos a la sierra... = If we go to the mountains...
• not si vayamos

The subjunctive does not usually appear directly after si in this kind of ordinary if clause.

Compare:

• Si tengo tiempo, voy. = If I have time, I’m going.
• Si llueve, nos quedamos en casa. = If it rains, we stay home.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• vamos already means we go / we are going

So Si vamos a la sierra... is completely natural. You could say Si nosotros vamos..., but it would usually sound unnecessary unless you want emphasis or contrast, for example:

• Si nosotros vamos, ellos también deberían ir.

In Spain, la sierra usually means a mountain area or mountain range, especially a natural inland area people might visit for hiking, skiing, snow, or countryside trips.

It is not exactly the same as just mountains in every context, but that is often the best natural English translation.

For example, in Spain people commonly say things like:

• Vamos a la sierra el fin de semana.

This often suggests going up to a mountain region, not necessarily to one specific peak.

Because Spanish usually uses en with seasons when speaking generally.

• en invierno = in winter
• en verano = in summer

This means winter in general, not one specific winter. It works like:

• En invierno hace frío.
• En verano vamos a la playa.

You may sometimes hear en el invierno de 2023 when referring to a specific winter, but here the general form en invierno is the normal choice.

Más vale is a fixed expression meaning something like:

• it’s better to...
• you’d better...
• it’s best to...

So:

• Más vale llevar ropa de abrigo = You’d better bring warm clothes / It’s better to wear warm clothing

Grammatically, vale is the third person singular of valer, but in this expression it behaves as an impersonal set phrase. You do not change it to match we.

You can also see:

• Más vale llegar pronto.
• Más vale prevenir que curar.

It is usually stronger than a mild suggestion. It often sounds like practical advice, a recommendation based on common sense, or even a warning.

So in this sentence, the speaker is not just casually commenting. They are basically saying:

• It would be wise to take warm clothes
• You really should take warm clothes

It is not necessarily rude or harsh, but it carries a sense of good idea / best not risk it.

In Spanish, llevar usually means to take / carry something to a place, while traer usually means to bring something toward the speaker or toward the place being discussed as a reference point.

Here, the idea is that we are going to the mountains, so we would take warm clothes with us:

• llevar ropa de abrigo

English often uses bring in places where Spanish prefers llevar, so this is a very common learner question.

Compare:

• Voy a llevar comida a la playa. = I’m going to take food to the beach.
• ¿Puedes traerme agua? = Can you bring me water?

Ropa de abrigo means warm clothes or outerwear for keeping warm.

The phrase de abrigo literally suggests clothing for warmth / for shelter from the cold. It can include things like:

• coats
• jackets
• scarves
• gloves
• warm layers

This is a fixed and very common expression in Spanish. You may also hear:

• un abrigo = a coat
• ponte ropa de abrigo = put on warm clothes

Because Spanish often uses noun + de + noun structures where English might prefer an adjective.

So instead of something like warming clothes or cold-weather clothes, Spanish says:

• ropa de abrigo

This kind of structure is very common:

• gafas de sol = sunglasses
• zapatos de deporte = sports shoes
• mesa de madera = wooden table

So de abrigo describes the function of the clothing: clothing for keeping warm.

Yes. That word order is possible and still natural.

The original version:

• Si vamos a la sierra en invierno, más vale llevar ropa de abrigo.

puts the condition first, which is very common because it sets the scene before giving the advice.

The alternative:

• Más vale llevar ropa de abrigo si vamos a la sierra en invierno.

puts the advice first.

Both are correct. The original just sounds especially natural because the speaker first mentions the situation, then the recommendation.

Yes, it is standard to use a comma when a conditional clause introduced by si comes first.

So this is the normal punctuation:

• Si vamos a la sierra en invierno, más vale llevar ropa de abrigo.

If the main clause comes first, the comma is usually not needed:

• Más vale llevar ropa de abrigo si vamos a la sierra en invierno.

Sometimes yes, but they are not always exactly the same.

• la montaña can refer more generally to the mountains or the mountainside / mountain area
• la sierra often suggests a mountain range or a specific kind of mountain region, especially in Spain

In everyday speech in Spain, ir a la sierra is a very natural expression for going to a mountain area for the day or weekend. Using la montaña might also work, but it can sound a bit more general depending on context.