Por muy rica que esté la tarta, yo prefiero algo más ligero, como una ciruela o unas frambuesas.

Por muy ... que is a concessive structure. It means something like:

• no matter how ...
• however ...
• even if it is very ...

So Por muy rica que esté la tarta means No matter how delicious the cake is or However delicious the cake may be.

It introduces a contrast: the cake may be delicious, but the speaker still prefers something lighter.

It is esté because por muy ... que normally triggers the subjunctive.

So:

• está = indicative
• esté = present subjunctive

The subjunctive is used here because the speaker is not simply stating a fact about the cake; they are presenting it as a concession: even if / no matter how delicious it is.

This pattern is very common:

• Por muy difícil que sea... = No matter how difficult it is...
• Por muy tarde que sea... = No matter how late it is...
• Por muy cansado que estés... = No matter how tired you are...

In Spanish, rico/rica often goes with estar when talking about how food tastes at that moment.

• La tarta está rica = The cake tastes delicious / is delicious
• La tarta es rica is much less natural here

With food, estar rico/a is the usual way to say something tastes good.

So rica is describing the cake’s condition or quality as food, not some essential identity.

Because rica agrees with la tarta, which is a feminine singular noun.

• el pastel → rico
• la tarta → rica

Spanish adjectives usually agree in gender and number with the noun they describe.

So:

• la tarta rica
• el plato rico
• las frambuesas ricas

In this structure, que links the intensified adjective phrase to the subjunctive clause.

The pattern is:

por muy + adjective/adverb + que + subjunctive

Examples:

• Por muy simpático que sea...
• Por muy lejos que esté...
• Por muy bien que cocine...

You should usually learn it as a fixed construction rather than translating each word separately.

No, yo is not strictly necessary, because Spanish verbs already show the subject:

• prefiero already means I prefer

So the sentence could simply be:

• Por muy rica que esté la tarta, prefiero algo más ligero...

Including yo adds a little emphasis or contrast, as if saying:

• As for me, I prefer...
• I, personally, prefer...

It helps highlight the speaker’s personal preference.

Algo is a very common pronoun meaning something.

So:

• algo más ligero = something lighter

It is much more natural than alguna cosa más ligera, which sounds more wordy and less idiomatic in most contexts.

Also, notice that algo is grammatically treated as masculine when followed by an adjective:

• algo bueno
• algo raro
• algo más ligero

That is why it is ligero, not ligera.

Because ligero agrees with algo, not with ciruela or frambuesas.

The structure is:

• prefiero algo más ligero = I prefer something lighter

Then como una ciruela o unas frambuesas gives examples of that something.

So the adjective matches algo, which takes masculine singular agreement in this kind of structure.

Because the speaker is giving examples, and the quantity is different:

• una ciruela = one plum
• unas frambuesas = some raspberries

Frambuesas are usually thought of as several small berries, so Spanish naturally uses the plural with unas in many contexts.

This sounds natural in the same way English often says:

• a plum
• some raspberries

In writing, yes, it looks the same, but here como means such as / like.

Compare:

• Como una ciruela = I eat a plum
• ..., como una ciruela o unas frambuesas. = ..., such as a plum or some raspberries.

The meaning is clear from context. In this sentence, como introduces examples.

Not really. Here por is part of the fixed expression por muy ... que. You should learn the whole chunk together.

Trying to translate por alone as for will usually not help here. The full structure means:

• however...
• no matter how...
• even if...

So it is better to recognize por muy ... que as one grammatical pattern.

In Spain, tarta often refers broadly to a cake or tart-type dessert, depending on context.

So in English, the best translation may vary:

• cake
• tart
• sometimes even dessert cake

If the meaning has already been given to the learner as cake, that is perfectly fine. But in real usage, tarta can be a little broader than just one exact English word.

Yes, it sounds natural in Spain.

The tone is:

• polite
• fairly neutral
• slightly refined or thoughtful

The structure Por muy ... que... is a bit more sophisticated than a very basic sentence like:

• La tarta está muy rica, pero prefiero algo más ligero.

So the original sentence sounds a little more elegant and expressive, but still completely normal.

Yes. Some simpler alternatives are:

• Aunque la tarta esté muy rica, yo prefiero algo más ligero.
• La tarta está muy rica, pero yo prefiero algo más ligero.

Differences:

• aunque + subjunctive gives a similar concessive idea
• the version with pero is simpler and more direct
• por muy ... que sounds a bit more advanced and emphatic: no matter how delicious the cake is...

So all are possible, but the original has a stronger even so feeling.