En el avión leí una revista cuya portada tenía una foto preciosa de la catedral.

Cuya means whose or of which.

So una revista cuya portada tenía... means a magazine whose cover had...

It is a possessive relative word: it links revista and portada and shows that the cover belongs to the magazine.

Because cuyo/cuya/cuyos/cuyas agrees with the noun that comes after it, not with the earlier noun.

Here, the noun after it is portada, which is:

• feminine
• singular

So the correct form is cuya.

Compare:

• el libro cuyo autor... → autor is masculine singular
• la revista cuya portada... → portada is feminine singular
• la casa cuyas ventanas... → ventanas is feminine plural

Because que by itself does not show possession.

Cuya specifically means whose / of which. It tells us that the portada belongs to the revista.

So:

• una revista cuya portada tenía... = a magazine whose cover had...

If you used que, you would need to restructure the sentence, for example:

• Leí una revista que tenía una foto preciosa en la portada.
• Leí una revista con una portada que tenía una foto preciosa de la catedral.

These are possible, but they are not exactly the same structure.

It is absolutely correct and native, but it often sounds a bit more formal or more written than some alternatives.

In everyday speech, many speakers might prefer a simpler version such as:

• Leí una revista que tenía una foto preciosa de la catedral en la portada.

So you should definitely learn cuyo/cuya, because it is standard and useful, but you should also know that spoken Spanish often avoids it.

Leí is the 1st person singular preterite of leer: I read.

The preterite is used because the sentence presents the reading as a completed event:

• En el avión leí una revista = On the plane I read a magazine

It refers to one finished action in the past.

Because tenía is the imperfect, and here it describes a background characteristic of the magazine’s cover.

The main event is:

• leí una revista

Then the sentence adds a description of that magazine:

• cuya portada tenía una foto preciosa...

The cover’s having that photo is not presented as a separate completed event. It is just a descriptive detail, so the imperfect sounds natural.

Also, tenía una foto here means something like:

• had
• contained
• displayed

You could sometimes hear a version with había, especially in a different structure:

• ...en cuya portada había una foto preciosa...

But tenía is very natural because it makes the cover the thing that had/displayed the photo.

The accent mark shows that the stress falls on the í:

• le-Í

It also marks a hiatus: the vowels are pronounced separately, not as one syllable.

So leí has two syllables:

• le-í

This is standard in forms of leer like:

• leí
• leía
• leíste

Because Spanish usually uses en with means of transport:

• en el avión = on the plane
• en el tren = on the train
• en el autobús = on the bus
• en el coche = in the car

Even though English often says on for public transport, Spanish normally uses en.

So En el avión is the normal way to say on the plane / while on the plane.

Portada means the front cover of a magazine, newspaper, or book.

In this sentence:

• la portada de la revista = the magazine cover

So cuya portada tenía una foto preciosa means the cover showed or contained a beautiful photo.

In Spanish, adjectives usually come after the noun, especially when they are simply describing it.

So:

• una foto preciosa = a beautiful photo

You can say una preciosa foto, but putting the adjective before the noun often sounds a bit more literary, expressive, or subjective.

So in a neutral sentence, foto preciosa is the most ordinary choice.

Here de means of in the sense of showing or depicting.

So:

• una foto de la catedral = a photo of the cathedral

This is the normal way to say what is in the picture.

The article la suggests a specific cathedral, probably one that is known from the context or obvious to the speaker and listener.

Compare:

• una foto de una catedral = a photo of a cathedral, any cathedral
• una foto de la catedral = a photo of the cathedral, a specific one