Nos refugiamos en la tienda de campaña cuando empezó a llover.

Because refugiarse is very commonly used as a pronominal verb in Spanish when it means to take shelter / to shelter oneself.

• nos = ourselves
• refugiamos = we sheltered / we took shelter

So nos refugiamos literally means we sheltered ourselves, though in natural English we usually say we took shelter.

Without nos, refugiamos would sound less natural here and could suggest that we sheltered someone or something else.

The preterite is used because this sentence describes a completed action in the past:

• Nos refugiamos... = We took shelter...
• empezó a llover = it started to rain

Both are seen as specific events that happened at a particular moment in the past.

If you used the imperfect instead, it would give a different feeling, such as background, habit, or an ongoing action. For example:

• Nos refugiábamos... would suggest we were taking shelter / we used to take shelter

But in your sentence, the speaker is talking about one definite event, so the preterite fits best.

Because after empezar, Spanish normally uses the preposition a before an infinitive:

• empezar a + infinitive

So:

• empezó a llover = it started to rain
• empezó a correr = he/she started to run
• empezamos a comer = we started eating

Leaving out a here would be incorrect in standard Spanish.

Because after empezó a, Spanish needs an infinitive:

• empezó a llover = it started to rain

Here, llover is not the main conjugated verb. The main conjugated verb is empezó.

Compare:

• Llovió = It rained
• Empezó a llover = It started to rain

So llover stays in the infinitive because it depends on empezó a.

Yes, tienda by itself usually means shop/store. That is why Spanish uses the full expression:

• tienda de campaña = tent

This is a fixed phrase. Literally, it looks like camping shop, but that is not how it is understood. As a whole, it simply means tent.

So:

• la tienda = the shop
• la tienda de campaña = the tent

You need the full expression to avoid confusion.

Spanish usually uses the definite article more often than English does. If the speaker is referring to a specific tent already known in the situation, la is natural:

• en la tienda de campaña = in the tent

In English, we may sometimes omit details more freely, but in Spanish the article is often required.

Without the article, en tienda de campaña would sound incomplete or unnatural in this sentence.

Because en is used for location inside a place, while a usually marks direction toward a place.

Here the idea is that they took shelter in the tent:

• en la tienda de campaña = in the tent

If you said a la tienda de campaña, that would suggest movement to the tent, not being inside it.

So compare:

• Fuimos a la tienda de campaña = We went to the tent
• Nos refugiamos en la tienda de campaña = We took shelter in the tent

Because cuando can be followed by different tenses depending on whether the action is in the past, present, or future.

Here the sentence tells a past event:

• cuando empezó a llover = when it started to rain

So the verb after cuando is also in a past tense.

Compare:

• Cuando empezó a llover, nos refugiamos... = past narration
• Cuando empiece a llover, nos refugiaremos... = future meaning, so subjunctive after cuando

In your sentence, both actions belong to a completed past situation.

Only empezó has a written accent in this sentence.

• empezó has an accent on ó because it is the 3rd person singular preterite form, and the accent helps show both pronunciation and verb form.
• refugiamos has no written accent.

This can be confusing because refugiamos can look like either:

• present: we shelter
• preterite: we sheltered

For many -ar verbs, the nosotros present and preterite forms are identical in spelling:

• hablamos = we speak / we spoke
• refugiamos = we shelter / we sheltered

Context tells you which meaning is intended. In your sentence, the past context makes it clearly preterite.

No. The correct form is refugiamos, with -ia-, because it comes from the infinitive refugiarse.

The verb is:

• refugiarse

So the nosotros preterite form is:

• nos refugiamos

A form like refugiamos would not match the regular conjugation pattern of refugiar.

Yes, it could be reversed. Spanish allows flexible word order here.

Your sentence:

• Nos refugiamos en la tienda de campaña cuando empezó a llover.

Also possible:

• Cuando empezó a llover, nos refugiamos en la tienda de campaña.

Both are correct. The difference is mostly one of focus and style:

• Starting with Nos refugiamos... puts the main action first.
• Starting with Cuando empezó a llover... sets the time/background first.

Both are very natural.

Yes. Llover is typically an impersonal verb in Spanish, just like to rain in English.

That means it is normally used only in the 3rd person singular:

• llueve = it rains
• llovió = it rained
• empezó a llover = it started to rain

There is no real subject person doing the action in the normal sense. Spanish does not need to say it explicitly, so you just get llover / llueve / llovió.

Yes. Depending on the context, Spanish speakers might also use:

• cobijarse
• resguardarse
• ponerse a cubierto

But refugiarse is very clear and natural here.

Each option has a slightly different feel:

• refugiarse = neutral, common, clear
• resguardarse = protect oneself from weather/danger
• cobijarse = shelter oneself, sometimes slightly more literary or regional

For a learner, nos refugiamos is an excellent and very usable choice.