Ayer iba a empezar un documental sobre volcanes, pero se fue la luz.

Iba a empezar literally means “was going to start”.
In Spanish, ir (in the imperfect) + a + infinitive is used to talk about:

• an intention or plan in the past
• that did not actually happen (or was interrupted)

So:

• Ayer iba a empezar un documental…
  = Yesterday I was going to start a documentary… (but something prevented it)

If you said:

• Ayer empecé un documental…
  = Yesterday I started a documentary… (you actually started it)

• Ayer empezaba un documental…
  = Yesterday I was starting / used to start a documentary… (describes an ongoing/habitual action; the interruption idea is weaker or unclear)

Here, the speaker wants to highlight a plan that didn’t happen because the power went out, so iba a empezar is the natural choice.

Iba a + infinitive is the past form of the “going to” future:

• Voy a empezar = I’m going to start
• Iba a empezar = I was going to start

Uses:

1. Past intention / plan:

   • Iba a llamar a mi hermana.
     = I was going to call my sister.

2. Plan interrupted or not carried out (very common):

   • Iba a salir, pero empezó a llover.
     = I was going to go out, but it started raining.

In your sentence, Ayer iba a empezar un documental… clearly sets up a planned action that got interrupted by the power outage.

The subject is understood from the verb form:

• iba is 1st person singular (yo) or 3rd person singular (él/ella/usted), so context tells you which.

In this sentence, it’s natural to understand:

• (Yo) iba a empezar un documental…
  = I was going to start a documentary…

If it were someone else, you’d usually mention the subject:

• Mi papá iba a empezar un documental…
  = My dad was going to start a documentary…

Literally, se fue la luz is something like “the light went away”.

• fue is from ir (to go), past tense: went
• la luz = the light / the electricity
• se makes the verb pronominal and softens it, turning it into a very natural, idiomatic way to say “the power went out”.

So:

• Se fue la luz.
  = The power went out.

The se is not a separate “meaning word” you can translate here; it’s part of a very common pattern in Spanish where se + verb is used for events that “just happen,” often without focusing on who’s responsible:

• Se rompió el vaso. = The glass broke. (It got broken.)
• Se me olvidó. = I forgot (it slipped my mind).

No difference in meaning; both are fine and both mean “the power went out.”

• Se fue la luz.
• La luz se fue.

Spanish word order is more flexible than English. In everyday speech, Se fue la luz is extremely common in much of Latin America. La luz se fue is also natural.

Context and intonation usually matter more than the small word order change here.

In everyday Spanish, especially in Latin America:

• la luz literally = “the light”
• but colloquially it often means the electrical power / supply.

So:

• Se fue la luz.
  = The power went out.

This is similar to English saying “the lights went out” to mean “the electricity went out.” More formal or technical options in Spanish would be:

• Se fue la electricidad.
• Se cortó la energía eléctrica.

But in normal conversation, la luz is by far the most common.

Fue is the preterite of ir (to go), and the preterite is used for completed events in the past.

• The power going out is seen as a sudden event: it happened, and it’s done → preterite.
  Se fue la luz. = The power went out.

Using iba (imperfect) here would be strange:

• Se iba la luz suggests “the power kept going out,” “the power was going out (repeatedly)” or a continuous background action.
  It’s possible in some contexts, but not for a single, one-time power cut.

So se fue la luz fits because it’s a single, completed incident.

Yes, that’s perfectly correct, and very natural:

• Ayer iba a empezar un documental sobre volcanes cuando se fue la luz.

Nuance:

• … pero se fue la luz.
  = “… but the power went out.”
  Emphasizes the contrast: intention vs. problem.

• … cuando se fue la luz.
  = “… when the power went out.”
  Emphasizes the moment of interruption.

Both are good; pero sounds a bit more like “I had this plan but something got in the way,” while cuando paints the scene more as timing/sequence.

Un documental = a documentary (non-specific, any documentary)
El documental = the documentary (a specific one both speaker and listener know)

In the sentence:

• Ayer iba a empezar un documental sobre volcanes…

we don’t need the listener to know exactly which documentary it was; the type of program is enough. So Spanish uses the indefinite article un.

If both people knew exactly which documentary, you might hear:

• Ayer iba a empezar el documental sobre volcanes, pero se fue la luz.
  = Yesterday I was going to start the volcano documentary, but the power went out.

Grammatical gender in Spanish is often arbitrary and must be memorized:

• el documental / un documental (masculine)
• la película (feminine)

The ending -al doesn’t automatically indicate gender. Many -al nouns are masculine:

• el hospital, el animal, el festival, el capital (money sense)

So you simply need to learn:

• documental → masculine noun → el / un documental

Yes, it can move without changing the meaning:

• Ayer iba a empezar un documental…
• Iba a empezar un documental ayer…

Both mean “Yesterday I was going to start a documentary…”

In Spanish, time expressions like ayer, hoy, mañana commonly go:

• at the beginning (for emphasis or style), or
• at the end of the clause.

Starting with Ayer is very natural storytelling style: it sets the time frame right away.

All three can express the idea of “about volcanoes”, but they have slightly different flavors and frequencies.

• sobre volcanes
  Very common in neutral language for topics / subjects:

  • un libro sobre volcanes
  • un documental sobre volcanes

• de volcanes
  Also possible, especially in shorter phrases; often a bit more general:

  • un documental de volcanes (okay, but many speakers prefer sobre here)

• acerca de volcanes / acerca de los volcanes
  A bit more formal or written-sounding:

  • un documental acerca de los volcanes

In this sentence, sobre volcanes is the most natural and typical choice for “a documentary about volcanoes.”

Yes: learners sometimes confuse iba with IVA.

• iba (with b) = from ir (yo/él/ella/usted iba) → was going
• IVA (with v, often capitalized) = Value Added Tax (Impuesto al Valor Agregado)

Pronunciation:

• iba = “EE-ba”
  Spanish b and v are pronounced almost the same, but spelling matters.

In this sentence you must use iba (from ir), not iva.