Creí que la superficie del planeta sería lisa, pero en las fotos se ven muchos detalles.

The verb creer is in the preterite: Creí = I thought / I believed (at that moment).

• Creí (preterite) refers to a finished, specific moment in the past:

  • Creí que la superficie del planeta sería lisa…
    At one point in the past, I had this belief.

• Creía (imperfect) would describe a habitual or long‑lasting belief:

  • Creía que la superficie del planeta era lisa.
    I used to think / I would think / I believed for a long time that it was smooth.

Here, the sentence talks about a specific belief that is later corrected by the photos, so the completed‑event feel of Creí fits better than the ongoing background belief of Creía.

In Spanish, verbs of thinking, saying, feeling, etc. usually need que to introduce the clause that follows.

• Creí que la superficie del planeta sería lisa…
  Literally: I thought *that the surface of the planet would be smooth…*

Without que, the sentence would be ungrammatical:

• ❌ Creí la superficie del planeta sería lisa… (wrong)

This pattern is very common:

• Pienso que… – I think that…
• Dijo que… – He/She said that…
• Sé que… – I know that…

Sería is the conditional of ser, and here it has a “future in the past” / expectation meaning:

• Creí que la superficie del planeta sería lisa
  I thought the surface of the planet would be smooth.

You’re talking about a belief you had back then about what the surface was going to be like (from your past point of view).

Other options change the meaning:

• Creí que la superficie del planeta era lisa.
  I thought the surface of the planet was smooth.
  → This sounds more like you thought it was already known to be smooth at that time (less about a prediction, more about a statement of fact).

• Creo que la superficie del planeta es lisa.
  Present: I think the surface of the planet is smooth.
  → That’s about your current belief, not a belief you had in the past.

So sería matches the idea “would be” (expectation or prediction seen from a past moment).

Grammatically, fuera (imperfect subjunctive) is possible in some contexts, but here it would sound odd or at least unusual to most speakers.

Main differences:

• Creí que… sería lisa.
  → Very natural. Sería expresses future in the past / expectation: what you imagined the surface would be like.

• Creí que… fuera lisa.
  → Fuera is subjunctive. This tends to suggest:

  • unreal or hypothetical situations (Quería que fuera lisa – I wanted it to be smooth), or
  • certain reported speech or doubts, often with a nuance of distrust, unreality, or evaluation.

With creer in the affirmative and a clear “expectation about the future”, Spanish strongly prefers the conditional: sería. So in this exact sentence, sería lisa is the natural choice.

Spanish normally uses a definite article (el, la, los, las) where English often omits “the”.

• la superficie del planeta = the surface of the planet
  This is a specific, unique surface (the surface that belongs to that planet), so Spanish marks that with la.

If you say just superficie del planeta, it sounds incomplete or title‑like, not like normal spoken or written Spanish. In normal sentences, you need:

• la superficie del planeta
• la atmósfera de la Tierra
• la temperatura del agua, etc.

In Spanish, every noun has a grammatical gender that you have to memorize. The word superficie happens to be feminine:

• la superficie – the surface
• una superficie lisa – a smooth surface

That’s why related words agree in gender:

• la superficie lisa (feminine singular)
• las superficies lisas (feminine plural)

There is no simple rule here; it’s just that superficie is classified as feminine in Spanish.

Del is the contraction of de + el:

• de = of / from
• el = the (masculine singular)
• de + el → del

So:

• la superficie del planeta = the surface of the planet

You must contract de + el → del:

• ✅ del planeta
• ❌ de el planeta

But you do not contract with other forms of el/la:

• de la luna (not dla)
• de los planetas (not dlos)
• de las estrellas (not dlas)

Lisa is an adjective meaning smooth that must agree in gender and number with the noun it describes.

• Noun: superficie – feminine singular
• Adjective: lisa – feminine singular form of liso/lisa

Examples:

• una superficie lisa – a smooth surface
• una mesa lisa – a smooth table
• un piso liso – a smooth floor
• dos superficies lisas – two smooth surfaces

So liso becomes lisa here because superficie is feminine.

This is a “se passive” / impersonal construction, very common in Spanish.

• En las fotos se ven muchos detalles.
  Literally: In the photos many details are seen.
  Natural English: In the photos you can see a lot of details / Many details can be seen in the photos.

Breaking it down:

• se: marks a passive‑like or impersonal structure.
• ven: 3rd person plural of ver (they see / are seen). It agrees with detalles.
• muchos detalles: the thing that is seen (grammatical subject in Spanish).

Pattern:

• Se ven muchos detalles. – Many details can be seen.
• Se ve un detalle. – One detail can be seen.
• Se venden casas. – Houses are sold / Houses for sale.
• Se habla español. – Spanish is spoken.

Spanish often uses se + 3rd person verb instead of a direct passive or instead of saying “one/you/people” as the subject.

The verb must agree with the thing that is seen:

• Subject: muchos detalles → plural
• Verb: ven (3rd person plural of ver)

So:

• En las fotos se ven muchos detalles. ✅
  In the photos, many details are seen / you can see many details.

If the subject were singular, you’d use se ve:

• En las fotos se ve un cráter. – You can see a crater in the photos.
• Desde aquí se ve la luna. – You can see the moon from here.

Both pero and sino can be translated as but, but they aren’t interchangeable.

• Pero = but in general, just introducing a contrast:

  • Creí que la superficie sería lisa, pero se ven muchos detalles.
    I thought it would be smooth, but you can see many details.
    → First idea vs. reality; not a direct “not X but Y” correction.

• Sino is used mainly in a “not X, but Y instead” structure:

  • No es lisa, sino rugosa. – It’s not smooth, but rough.
  • No quería agua, sino café. – I didn’t want water, but coffee.

In the original sentence, there is no “no X, sino Y” contrast stated. You’re not saying “It’s not smooth, but detailed instead”; you’re saying “I thought X, but in fact Y”. That’s a job for pero.

In Spanish, the verb ending shows the subject, so the subject pronoun (yo, tú, él, etc.) is often dropped.

• Creí already tells us it’s 1st person singular (I).
  So yo is not necessary:
  • Creí que… = I thought that…

You can say:

• Yo creí que la superficie del planeta sería lisa…

This is grammatically correct, but yo adds emphasis, like:

• I thought it would be smooth (maybe others didn’t).

Neutral, normal version: Creí que…
Emphatic or contrastive: Yo creí que…

Yes, detalles is very similar to English “details”, but here it refers specifically to visible features of the planet’s surface:

• muchos detalles = a lot of detail / many features
  Things like craters, mountains, ridges, textures, etc., that you can distinguish in the photos.

So:

• Creí que la superficie del planeta sería lisa, pero en las fotos se ven muchos detalles.
  → I thought the planet’s surface would be smooth, but in the photos you can see lots of visible features / a lot of texture.