Si hace viento, es mejor refugiarse detrás de una piedra grande o dentro de una cueva.

In Spanish, weather is often expressed with hacer:

• hace frío = it’s cold
• hace calor = it’s hot
• hace sol = it’s sunny
• hace viento = it’s windy

So si hace viento is the most natural everyday way to say if it’s windy.

You can say es ventoso, but it sounds more descriptive or formal, and it is less common in ordinary conversation.

No. si without an accent means if.

• Si hace viento... = If it’s windy...

But sí with an accent means yes or sometimes indeed.

• Sí, claro. = Yes, of course.

This is a very common spelling distinction in Spanish.

Spanish often uses es mejor + infinitive to say it is better to...

So:

• es mejor refugiarse = it’s better to take shelter
• literally: it is better to shelter oneself

This is a very common structure:

• Es mejor esperar. = It’s better to wait.
• Es mejor salir temprano. = It’s better to leave early.

You do not need que here because the sentence is not saying it is better that someone does something. It is saying it is better to do something in general.

Refugiarse is the usual verb for to take shelter / to shelter oneself.

The -se makes it reflexive, meaning the subject is doing the action to themselves:

• refugiarse = to take shelter
• literally, to shelter oneself

In this sentence, that is the natural choice, because the idea is that a person protects themselves from the wind.

Compare:

• refugiarse en una cueva = to take shelter in a cave
• refugiar a alguien = to shelter someone

So without -se, the verb would usually need an object.

In Spanish, when a verb is in the infinitive, gerund, or affirmative command form, object and reflexive pronouns can attach to the end.

So:

• refugiarse = to shelter oneself
• lavarse = to wash oneself
• ponerse = to put on

Here, because the verb is an infinitive after es mejor, attaching -se is completely normal.

You could also sometimes restructure the sentence in other contexts, but in this sentence es mejor refugiarse is the standard form.

Detrás de means behind.

In Spanish, many location words work as fixed expressions with de:

• detrás de = behind
• delante de = in front of
• encima de = on top of
• debajo de = under / underneath

So you say:

• detrás de una piedra = behind a rock

You normally cannot say just detrás una piedra.

Both are possible, but they are slightly different in feel.

• dentro de una cueva = inside a cave
• en una cueva = in a cave

Dentro de emphasizes being inside the cave more clearly. Since the sentence is about protection from the wind, that sense of being properly inside is helpful.

So dentro de una cueva sounds a bit more precise than en una cueva.

Because they express two different kinds of location:

• detrás de una piedra grande = behind a large rock
• dentro de una cueva = inside a cave

A rock is something you can hide behind, while a cave is something you can go inside. Spanish is being natural and specific here, just like English would.

In Spanish, adjectives usually come after the noun:

• una piedra grande = a big/large rock
• una cueva pequeña = a small cave

Putting the adjective before the noun is sometimes possible, but it often changes the tone or sounds literary, subjective, or unusual.

So for a normal physical description, piedra grande is the expected order.

Grande usually becomes gran before a singular noun:

• un gran hombre
• una gran idea

But the meaning can shift.

• una piedra grande usually means a large rock in physical size
• una gran piedra can also mean a large rock, but it may sound a bit more expressive or less neutral

In a practical sentence like this, una piedra grande is the most straightforward choice because it clearly refers to physical size.

Normally, o means or.

Spanish changes o to u only before words that begin with an o sound, to avoid repetition of the same sound:

• siete u ocho = seven or eight
• padres u otros familiares = parents or other relatives

But here the next word is dentro, which does not begin with an o sound, so o stays the same:

• ...una piedra grande o dentro de una cueva

Because this is a general real condition: if it’s windy, it’s better...

Spanish uses the present tense after si for real, possible, or general situations:

• Si llueve, nos quedamos en casa.
• Si hace frío, ponte un abrigo.

So si hace viento is exactly what you would expect for a real present condition.

Spanish does not use a future tense after si in this type of sentence.

Normally, no.

After si, Spanish usually does not use the future tense for real future conditions. Instead, it uses the present tense:

• Si mañana hace viento, nos quedamos dentro.
• literally: If tomorrow it is windy, we’ll stay inside.

So even when English says if it will be windy, Spanish generally says si hace viento.

Piedra can mean stone or rock, depending on context.

In this sentence, English speakers will often think of a large rock / boulder-like stone because the person is taking shelter behind it. The exact English word depends on context, but the Spanish word piedra is perfectly natural here.

Because hacer + weather noun works as a set expression in Spanish.

So you say:

• hace viento
• hace sol
• hace frío

not normally:

• hace el viento

In this structure, the weather noun appears without an article.

Yes, si hay viento is possible and understandable, but si hace viento is more idiomatic for general weather conditions.

A useful rough distinction is:

• hace viento = it’s windy
• hay viento = there is wind

Both can work, but hace viento is the usual weather expression.

Yes, cueva is a feminine noun:

• la cueva
• una cueva

That is why the article is una, not un.

Its adjective would also agree if needed:

• una cueva oscura = a dark cave

Sometimes, depending on context, but literally es mejor means it’s better.

• Es mejor refugiarse... = It’s better to take shelter...

If you want it’s best, Spanish often uses:

• lo mejor es...
• es lo mejor...

For example:

• Lo mejor es refugiarse en una cueva. = The best thing is to take shelter in a cave.

In your sentence, es mejor gives practical advice rather than saying it is the absolute best option.

Because Spanish often leaves the subject unstated when it is general or obvious from context.

Here, the sentence gives general advice:

• es mejor refugiarse... = it’s better to take shelter...

It is similar to English it’s better to..., where no specific person is named. Spanish can do this very naturally with es mejor + infinitive.

Yes, but it would change the tone.

• Es mejor refugiarse... = It’s better to take shelter...
  
  • general advice
• Debes refugiarte... = You should / must take shelter...
  
  • more direct, stronger instruction

The original sentence sounds more neutral and general, which is often preferred in advice or instructions.

It sounds natural in Spain and also in most other Spanish-speaking regions. The vocabulary and grammar are standard:

• si hace viento
• es mejor
• refugiarse
• detrás de
• dentro de una cueva

Nothing here is strongly regional. A speaker from Spain would find it completely normal, and so would many speakers elsewhere.