El yogur está demasiado agrio para mí, pero a mi hermana le gusta porque el chocolate ya es bastante amargo.

Spanish often uses estar for a condition or state, especially when something tastes, smells, or feels a certain way at that moment.

• El yogur está agrio = The yogurt tastes/is sour.
• El chocolate es amargo = Chocolate is bitter by nature.

So in this sentence:

• El yogur está demasiado agrio suggests the yogurt is sour in its current state.
• El chocolate ya es bastante amargo treats bitterness as a normal characteristic of chocolate.

This is a very common ser vs estar contrast:

• estar = temporary state, condition, result
• ser = essential characteristic, general nature

They are not the same taste.

• agrio = sour, acidic
  
  • like spoiled milk, plain yogurt, lemon gone sharp
• amargo = bitter
  
  • like dark chocolate, coffee, some medicines

So:

• yogur agrio = sour yogurt
• chocolate amargo = bitter chocolate

English learners often confuse them because both are “unpleasant” tastes in some contexts, but Spanish keeps them clearly separate.

Here demasiado means too.

• demasiado agrio = too sour

It intensifies the adjective and shows excess:

• demasiado caliente = too hot
• demasiado caro = too expensive

So El yogur está demasiado agrio para mí means the yogurt is more sour than I can comfortably enjoy.

Because para mí means for me / to me / in my opinion or according to my taste.

In this sentence:

• demasiado agrio para mí = too sour for me

That means the yogurt is too sour for my taste, not necessarily for everyone.

Por mí would mean something very different, such as:

• Por mí, está bien = As far as I’m concerned, it’s fine / For me, it’s fine

So here para mí is the natural choice.

Because gustar does not work like English to like.

Spanish structures it more like:

• To my sister, it is pleasing

So:

• A mi hermana le gusta = My sister likes it

The person who experiences the liking is expressed with:

• a + person and also usually with an indirect object pronoun:
• me, te, le, nos, os, les

Examples:

• Me gusta = I like it
• A Juan le gusta = Juan likes it
• A mis padres les gusta = My parents like it

So mi hermana gusta is incorrect because gustar needs that indirect-object structure.

It is a normal and very common feature of Spanish called clitic doubling.

• le already means to her
• a mi hermana specifies exactly who that le refers to

So:

• Le gusta = She likes it / He likes it / You like it
• A mi hermana le gusta = My sister likes it

Spanish often keeps both:

1. the pronoun (le)
2. the clarifying phrase (a mi hermana)

This is especially common with gustar and similar verbs.

Here ya adds the idea of already / as it is / anyway.

So:

• el chocolate ya es bastante amargo means something like
  chocolate is already quite bitter as it is

It suggests that bitterness is already part of chocolate’s nature, so maybe extra sourness or sharpness is not a problem for the sister.

Depending on context, ya can have several shades of meaning:

• time: already
• emphasis: as it is
• change of state: now

In this sentence, as it is already is the best way to understand it.

Here bastante means quite / fairly / rather.

• bastante amargo = quite bitter / fairly bitter

It can sometimes mean enough, but that depends on context.

Compare:

• Es bastante amargo = It’s quite bitter
• Ya tengo bastante = I already have enough

So in this sentence, bastante is an adverb modifying amargo, not a stand-alone pronoun meaning enough.

Because they agree with the nouns they describe:

• el yogur → masculine singular → agrio
• el chocolate → masculine singular → amargo

In Spanish, adjectives usually agree in gender and number with the noun.

Examples:

• el yogur agrio
• la leche agria
• los limones agrios
• las naranjas agrias

The same pattern applies to amargo:

• el chocolate amargo
• la medicina amarga

Spanish often leaves the subject understood from context.

In a mi hermana le gusta, the thing that is liked is understood from what comes before: probably the yogurt or perhaps that fact/flavor depending on context.

Spanish does this a lot:

• Me gusta = I like it
• No le gusta = He/She doesn’t like it

If you wanted to make it explicit, you could say:

• A mi hermana le gusta el yogur
• A mi hermana le gusta eso

But when context is clear, Spanish often omits the noun or pronoun.

Because with ser, the noun being described is the grammatical subject.

• El chocolate es bastante amargo
  = Chocolate is quite bitter

Here:

• el chocolate = subject
• es = verb
• bastante amargo = description

This part is more similar to English than the gustar part is.

So the sentence switches between:

1. a normal descriptive structure: El yogur está...
2. a gustar structure: a mi hermana le gusta
3. another normal descriptive structure: el chocolate ya es...

Yogur is very common in Spain, and it is a standard Spanish spelling. You may also see yogurt in some places.

In Spain, yogur is especially normal and common in everyday usage.

So for a learner of Spanish from Spain, el yogur looks perfectly natural.

Yes, but the meaning would change.

• bastante agrio = quite sour / fairly sour
• demasiado agrio = too sour

So:

• El yogur está bastante agrio is more neutral: it describes the taste.
• El yogur está demasiado agrio para mí adds a negative judgment: it exceeds what the speaker likes.

That contrast is important in this sentence, because the speaker dislikes the level of sourness, while the sister is okay with it.