Learn Hardcore Spanish (Spain): Anoche tuve una pesadilla en la que perdía el pasaporte en el aeropuerto. - Last night I had a nightmare in which I lost my passport at the airport.

Tuve = a single, completed event, seen as a whole:
- Anoche tuve una pesadilla.
  “Last night I had a nightmare (one specific event).”
Tenía = a repeated or ongoing situation, background description:
- Cuando era pequeño, tenía muchas pesadillas.
  “When I was little, I used to have a lot of nightmares.”

In your sentence, it’s one specific nightmare that happened last night, so tuve is the natural choice.

Why is it perdía (imperfect) and not perdí (preterite)?

Inside the dream, the speaker uses the imperfect (perdía) to present the action as ongoing, incomplete, or part of the “background” of the nightmare.

perdía el pasaporte suggests:
- you were in the process of losing it,
- or you kept losing it / couldn’t keep track of it,
- or the focus is on the situation, not on the final result.

If you said:

…en la que perdí el pasaporte…

it would sound more like a single, completed fact inside the dream: “in which I lost the passport (and that’s it)”. It’s not wrong, but perdía feels more like you’re inside the nightmare, living that stressful situation.

Spanish often uses the imperfect in dream descriptions to give that “ongoing / scene-like” feeling:

Soñé que iba en un tren y perdía el pasaporte.
“I dreamed I was on a train and (kept) losing my passport.”

In English we say “a nightmare where I lost my passport”. Why is it en la que and not donde?

En la que literally means “in which” and is the most straightforward, standard option here.
La que is a relative pronoun referring back to la pesadilla (feminine singular), and en comes from the idea “in that nightmare”.

Donde is normally used for places (“where” in a spatial sense):

El aeropuerto donde perdí el pasaporte.
“The airport where I lost my passport.”

A pesadilla is not a physical place, so en la que is more precise and more standard. People sometimes stretch donde to non-places in casual speech, but en la que is the safest, most natural choice here.

What exactly does la que refer to in en la que?

La que refers back to una pesadilla:

pesadilla is feminine and singular → la que (not el que, los que, etc.).

So you can think of it as:

una pesadilla [en la pesadilla perdía el pasaporte]
→ una pesadilla en la que perdía el pasaporte
“a nightmare in which I was losing my passport”

Could I say Anoche soñé que perdía el pasaporte en el aeropuerto instead? Is there a difference?

Yes, that’s perfectly correct and very natural.

Nuance:

Tuve una pesadilla…
Emphasises that it was a nightmare (unpleasant, scary dream).
Soñé que…
Is neutral: just “I dreamed that…”, which could be good or bad.

So:

Anoche tuve una pesadilla en la que perdía el pasaporte…
→ focuses on the fact it was a nightmare.
Anoche soñé que perdía el pasaporte…
→ simply states what you dreamed, without highlighting “nightmare” so strongly.

Can I say en que instead of en la que?

Yes, en que is also used, especially in spoken Spanish:

Anoche tuve una pesadilla en que perdía el pasaporte…

Differences:

en la que is a bit clearer and more explicit, because la shows the gender/number and clearly links back to pesadilla.
en que is slightly more informal and a bit more compact.

Both are accepted in Spain; en la que is the safest option in careful or written Spanish.

Why is it el pasaporte and not just pasaporte or mi pasaporte?

In Spanish, the definite article is often used where English would use “my” or no article:

Perdí el pasaporte.
Usually understood as “I lost my passport.”

The context already makes it obvious whose passport it is, so mi pasaporte isn’t necessary (though it’s not wrong).

Using el pasaporte instead of bare pasaporte is also normal because Spanish rarely uses a singular count noun without an article. You practically always need un, el, mi, etc.

Why en el aeropuerto and not en un aeropuerto?

Both are possible, but they suggest slightly different things:

en el aeropuerto
→ “at the airport” (a specific airport that’s understood from context: the one you’re travelling from/to, or the generic “airport situation” of the dream).
en un aeropuerto
→ “at an airport” (some unspecified airport, not important which one).

In a typical travel nightmare, speakers instinctively say en el aeropuerto, because in their mind it’s the airport they’re travelling through in the dream, even if they never name it.

Is pesadilla always feminine? What about pasaporte?

Yes, pesadilla is feminine:

una pesadilla terrible
la pesadilla
las pesadillas

Pasaporte is masculine:

un pasaporte
el pasaporte
los pasaportes

Gender is mostly something you memorise. A lot of -a words are feminine and many -e words are masculine, but there are exceptions, so it’s safest to learn new nouns together with el / la.

Can I move anoche to the end: Tuve una pesadilla anoche…? Is word order flexible?

Yes, that’s fine:

Anoche tuve una pesadilla…
Tuve una pesadilla anoche…

Both are natural. Differences:

Anoche tuve… puts a little more emphasis on when it happened.
Tuve una pesadilla anoche… puts the focus first on the fact you had a nightmare, then adds when.

In everyday speech, both orders are used freely.

What’s the difference between anoche and ayer por la noche?

They’re very close in meaning:

anoche = last night (always the night immediately before now).
ayer por la noche = yesterday evening / last night.

In most contexts they’re interchangeable. Anoche is shorter and more common; ayer por la noche is just a bit more explicit, and can sometimes sound slightly more formal or careful.