En el documental vimos un volcán junto al océano y luego hablamos del desierto.

En el documental means in the documentary or during the documentary, depending on context.

Here it tells you the setting or context for what happened:

• En el documental vimos... = In the documentary, we saw...

Spanish often uses en in cases where English might use in, on, or even during.
You could also say Durante el documental... if you want to emphasise during the documentary, but en el documental is very natural when talking about what appeared in it.

Both vimos and hablamos are in the preterite, the tense used for completed actions in the past.

• vimos = we saw
• hablamos = we talked

The sentence describes two completed events:

1. first, we saw a volcano
2. then, we talked about the desert

That is exactly the kind of sequence where the preterite is normally used.

In Spanish, subject pronouns are often left out because the verb ending already tells you who the subject is.

• vimos = we saw
• hablamos = we talked

The -mos ending shows the subject is nosotros / nosotras (we).
So Spanish does not need to include nosotros unless the speaker wants emphasis or contrast.

This is about articles.

• un volcán = a volcano
  This introduces something not previously identified: just a volcano.

• al océano = next to the ocean
  Here el océano uses the definite article the.

• del desierto = about the desert
  Again, el desierto uses the.

Spanish uses the definite article more often than English does, especially with geographic features and general nouns. So el océano and el desierto sound very natural.

They are contractions:

• al = a + el
• del = de + el

So:

• junto al océano = junto a el océano → contracted to junto al océano
• hablamos del desierto = hablamos de el desierto → contracted to hablamos del desierto

These contractions are normally mandatory when a or de comes before el.

Exceptions:

• If el is part of a proper name, like El Escorial, you do not contract:
  • Voy a El Escorial
  • Vengo de El Escorial

Junto a means next to, beside, or close to.

So:

• junto al océano = next to the ocean

It is a common expression:

• junto a la playa = next to the beach
• junto al río = beside the river

It is similar to cerca de (near), but junto a often suggests being very close, almost right beside something.

Luego means then or afterwards.

In this sentence:

• ...y luego hablamos del desierto = ...and then we talked about the desert

It marks the next event in the sequence.

Its position is flexible, but this placement is very natural:

• ...y luego hablamos...

You could also say:

• Luego hablamos del desierto
• Hablamos luego del desierto

But the version in the sentence is the most straightforward and common.

Spanish often uses the definite article where English may or may not use one.

Here:

• el océano
• el desierto

That sounds normal because Spanish likes using articles with many nouns, including natural features and general categories.

Compare:

• Vimos el mar = We saw the sea
• Hablamos del clima = We talked about the climate
• Estudiamos el español = We study Spanish

English sometimes drops the article more easily than Spanish does.

The accents show which syllable is stressed.

• volcán → stress on the last syllable: vol-CÁN
• océano → stress on the second syllable: o-CÉ-a-no

These written accents are important because they tell you how to pronounce the word correctly.

Without the accent, Spanish stress rules would suggest a different pronunciation.

This is a very common learner question.

• hablamos = we talked / we spoke
  This is preterite: a completed action.

• hablábamos = we were talking / we used to talk
  This is imperfect: ongoing, habitual, descriptive, or background action.

In your sentence, hablamos is used because the speaker is telling a sequence of completed events:

• we saw a volcano
• then we talked about the desert

If you said hablábamos, it would suggest an ongoing background action instead.

Yes, Spanish word order is somewhat flexible, although the original version is very natural.

Original:

• En el documental vimos un volcán junto al océano y luego hablamos del desierto.

Possible variations:

• Vimos en el documental un volcán junto al océano y luego hablamos del desierto.
• Luego hablamos del desierto después de ver un volcán junto al océano en el documental.

The original sounds good because it is clear and organised:

1. context: En el documental
2. first event: vimos...
3. second event: luego hablamos...

A speaker from Spain would typically pronounce it roughly like this:

• En el documental
• vimos un volcán junto al océano
• y luego hablamos del desierto

A few useful pronunciation points:

• j in junto sounds like a strong throaty sound, like the ch in Scottish loch
• c before e in océano is often pronounced th in much of Spain
• volcán is stressed on the last syllable
• océano is stressed on cé
• desierto has the stress on sier

So in much of Spain:

• océano sounds roughly like o-THE-a-no
• junto sounds roughly like HOON-to

Yes. Hablar de is the standard way to say to talk about.

Examples:

• Hablamos del desierto = We talked about the desert
• Hablo de mi trabajo = I talk about my job
• ¿De qué habláis? = What are you talking about?

So del desierto here comes from:

• hablar de + el desierto → hablar del desierto

Sometimes yes, but the meaning changes slightly.

• océano = ocean
• mar = sea

So:

• junto al océano = next to the ocean
• junto al mar = next to the sea

Both are natural Spanish, but they are not exactly the same thing. The sentence uses océano because that is the specific noun intended.