Cuando el arquitecto habló del ángulo del tejado, mi padre dijo que prefería verlo en un dibujo y no solo en una fórmula.

Del is the contraction of de + el.

So:

• del ángulo = de el ángulo = of the angle / about the angle
• del tejado = de el tejado = of the roof

In Spanish, de el normally contracts to del. The only common exception is when El is part of a proper name, such as de El Escorial.

So del ángulo del tejado literally breaks down as:

• del ángulo = about the angle
• del tejado = of the roof

Together: about the angle of the roof.

This is a very common contrast in Spanish.

• habló and dijo are in the preterite because they describe completed actions in the past.
• prefería is in the imperfect because it expresses an ongoing attitude, preference, or state at that moment.

So the idea is:

• The architect spoke about the roof angle.
• My father said something.
• What he said was that he preferred to see it in a drawing.

The preference is treated as a state or viewpoint, not as a single completed event.

English often uses the same form for all of these, but Spanish makes this distinction much more clearly.

Yes, it could, but the nuance changes.

• prefería verlo... = he preferred to see it...
  This sounds like his preference or attitude at that moment.
• prefirió verlo... = he chose / preferred to see it...
  This sounds more like a specific, completed decision.

In this sentence, prefería is more natural because the father is expressing a general preference in response to what the architect said.

Because habló refers to a specific completed event: the moment when the architect spoke.

• Cuando el arquitecto habló... = When the architect spoke...
• Cuando el arquitecto hablaba... would mean something more like While the architect was speaking...

So:

• habló = one whole finished action
• hablaba = ongoing background action

In this sentence, the father reacted after or at the point when the architect spoke, so habló fits better.

After verbs like decir, Spanish often uses que to introduce reported speech.

• Mi padre dijo que... = My father said that...

This is very normal Spanish structure.

Compare:

• Mi padre dijo que prefería verlo... = reported speech
• Mi padre dijo: Prefiero verlo... = direct speech

So que works just like that in English, although in English we often omit that, while in Spanish que is usually kept.

Verlo = ver + lo = to see it

The pronoun lo is a direct object pronoun meaning it.

Here, lo most naturally refers to the thing being discussed, probably el ángulo del tejado or the idea of that angle as represented visually.

Spanish often uses object pronouns attached to infinitives:

• verlo = to see it
• hacerlo = to do it
• comprarlo = to buy it

You could also say prefería verlo because after a conjugated verb plus an infinitive, the object pronoun can attach to the infinitive.

Because with an infinitive, Spanish allows object pronouns to be attached directly to it.

So these are both possible in many structures:

• prefería verlo
• lo prefería ver

But the first one, prefería verlo, is much more natural here.

This is a very common pattern after verbs like:

• querer → quiero verlo
• poder → puedo hacerlo
• necesitar → necesito comprarlo

For learners, it is often easiest to remember that pronouns commonly attach to infinitives.

Because hablar de is the most common and neutral way to say to talk about something.

• hablar de algo = to talk about something

So:

• habló del ángulo = he talked about the angle

Hablar sobre also exists, but it can sound a bit more formal, academic, or deliberate depending on context.

In everyday Spanish, hablar de is usually the most natural choice.

In Spain, tejado commonly refers to the outside roof of a house or building, especially a sloping roof.

• tejado = roof
• techo = ceiling, or sometimes roof in broader usage

For many learners, the key idea is:

• tejado often focuses on the external roof
• techo often means the inside ceiling

Since this sentence is about the angle of a roof in an architectural context, tejado is a very good choice, especially in Spain Spanish.

Here en means something like in or by means of / as represented in.

So:

• verlo en un dibujo = to see it in a drawing
• no solo en una fórmula = not only in a formula

The idea is that the father wants a visual representation, not just an abstract mathematical one.

This use of en is very natural in Spanish when talking about how something appears or is represented:

• Lo vi en un mapa = I saw it on a map
• Está en una foto = It’s in a photo
• Explícamelo en un esquema = Explain it to me in a diagram

Because Spanish often uses the definite article with professions or common nouns when talking about a specific person:

• el arquitecto = the architect

But possessives like mi, tu, su, etc. usually replace the article:

• mi padre = my father
• not el mi padre

So:

• el arquitecto = the architect
• mi padre = my father

That is standard Spanish grammar.

Modern standard Spanish usually writes solo without an accent, even when it means only.

So in this sentence:

• no solo en una fórmula = not only in a formula

Older usage often wrote sólo to distinguish it from solo meaning alone, but current spelling rules generally prefer solo without the accent unless there is real ambiguity.

So the spelling in the sentence is normal and correct.

They have accent marks because Spanish spelling shows stress very systematically.

A quick breakdown:

• ángulo: stress is on the first syllable, so it needs an accent
• habló: stress is on the last syllable, and the accent distinguishes it from hablo
• prefería: the accent shows that -ía forms separate syllables
• fórmula: stress is on the first syllable, so it needs an accent

Accent marks in Spanish are not optional decoration; they tell you how to pronounce the word and sometimes also help distinguish different verb forms.

For example:

• hablo = I speak
• habló = he/she spoke

So the accent in habló is especially important.