Mi hermano decía que la llanura era aburrida, pero cambió de opinión cuando vio el atardecer desde la carretera.

Because they describe a past state/background, not a single completed event.

• decía = he used to say / he was saying
• era aburrida = it was boring

In this sentence, the brother’s opinion is presented as an ongoing view he had in the past. Spanish often uses the imperfect for:

• habitual actions
• repeated statements
• descriptions
• background information

So Mi hermano decía que la llanura era aburrida sounds like My brother used to say that the plain was boring or My brother would say that the plain was boring.

Because those are completed actions that move the story forward.

• cambió de opinión = he changed his mind
• vio el atardecer = he saw the sunset

The preterite is used for specific events seen as finished. In this sentence, the structure is very natural:

• imperfect for the old situation/background: decía, era
• preterite for the key events/change: cambió, vio

This contrast is one of the most common storytelling patterns in Spanish.

Yes, but the meaning changes slightly.

• decía suggests a repeated or ongoing opinion: he used to say it, or that was his general view.
• dijo suggests one specific moment when he said it: he said it once.

So:

• Mi hermano decía que la llanura era aburrida = this was his usual opinion
• Mi hermano dijo que la llanura era aburrida = at one particular time, he said it

Both are grammatical, but decía fits better if you want to contrast an old attitude with a later change of mind.

Because aburrida here describes the plain as boring, not bored.

This is a classic ser vs estar point:

• ser aburrido/a = to be boring
• estar aburrido/a = to be bored

So:

• La llanura era aburrida = The plain was boring
• Mi hermano estaba aburrido = My brother was bored

In other words, the plain causes boredom, while the brother feels boredom.

Because it agrees with la llanura, which is a feminine noun.

In Spanish, adjectives usually agree in gender and number with the noun they describe:

• el libro aburrido
• la película aburrida
• los libros aburridos
• las películas aburridas

Here:

• la llanura = feminine singular
• so the adjective must be aburrida

Because cambiar de opinión is the normal Spanish expression for to change one’s mind.

It is a fixed phrase:

• cambiar de opinión = to change one’s mind/opinion

So cambió de opinión is the most natural choice.

You might hear cambiar su opinión in some contexts, but it is less idiomatic here. For a learner, it is best to remember:

• tener una opinión = to have an opinion
• cambiar de opinión = to change one’s mind

Because decía que is simply reporting what someone said or believed. It does not automatically trigger the subjunctive.

Here, the speaker is presenting the content of the brother’s statement as his opinion:

• Mi hermano decía que la llanura era aburrida

That is normal indicative usage.

The subjunctive is more likely after expressions of doubt, emotion, unreality, influence, and so on. But decir que in a straightforward reporting sense normally takes the indicative:

• Dijo que venía.
• Decía que era verdad.

So era is exactly what you would expect here.

La llanura means the plain: a large, flat area of land.

It is a geographical word, and it sounds a little more specific and descriptive than just saying el campo.

A rough comparison:

• la llanura = the plain / flatland
• el campo = the countryside
• la meseta = plateau
• la pradera = meadow / prairie, depending on context

So la llanura gives a stronger image of a wide, open, flat landscape.

They are related, but not exactly identical.

• el atardecer = dusk / sunset time / the evening light as the day ends
• la puesta de sol = the sunset, especially the actual setting of the sun

In many contexts they overlap, and both could work. But el atardecer often feels a bit broader and more atmospheric. It can refer to the whole moment or ambiance of late evening light, not just the exact instant the sun goes down.

So in this sentence, vio el atardecer suggests he saw that beautiful end-of-day scene, which then changed his opinion.

Here desde means from, in the sense of from the viewpoint of or from a position on the road.

So:

• vio el atardecer desde la carretera = he saw the sunset from the road

It suggests the road was the place from which he saw it.

Depending on context, it could mean:

• he was standing on the roadside
• he was travelling along the road
• the road gave him the view

Spanish often uses desde very naturally for a viewing point:

• desde la ventana = from the window
• desde el balcón = from the balcony
• desde la montaña = from the mountain

Not with the same meaning.

• cuando vio = when he saw it, at that specific moment
• cuando veía = when he was seeing / whenever he saw, which sounds ongoing or habitual

In this sentence, the idea is that one specific experience changed his mind. That is why the preterite vio is the natural choice.

So the timeline is:

1. he used to think the plain was boring
2. then one day he saw the sunset from the road
3. that experience changed his mind

That sequence strongly favors vio, not veía.