Después de otro sorbo, mi hermano decidió que el batido estaba mejor sin tanto sirope.

Because después normally needs de before a noun phrase or an infinitive.

• después de otro sorbo = after another sip
• después de comer = after eating

If there is no noun or infinitive after it, you can use después on its own:

• Después, mi hermano habló. = Afterwards, my brother spoke.

And if a full clause follows, Spanish often uses después de que:

• Después de que probó el batido...

So in this sentence, de is required because otro sorbo is a noun phrase.

Because otro already works as a determiner, so you do not normally say un otro sorbo.

• otro sorbo = another sip
• otra vez = another time / again
• otro día = another day

This is the normal pattern in Spanish. Otro changes for gender and number:

• otro sorbo
• otra cucharada
• otros sorbos
• otras bebidas

Sorbo means sip: a small amount of a drink taken into the mouth.

It is a very natural word here because a milkshake is something you sip. A related word is trago, but that can feel a bit broader and may also mean gulp or even an alcoholic drink, depending on context. In this sentence, sorbo is the best fit.

Because decidió presents the decision as a single, completed action in the past.

• decidió = he decided
• decidía = he was deciding / used to decide

Here, the brother reaches a conclusion at a particular moment, so the preterite is the natural choice.

The accent mark is also important:

• decidió is the correct past form
• it is stressed on the last syllable: de-ci-dió

Because decidió que... here means he came to the conclusion that... or decided that... in the sense of forming an opinion.

• Decidió que el batido estaba mejor... = He concluded that the milkshake was better...

If Spanish used decidió + infinitive, the meaning would be more about choosing to do an action:

• Decidió beberlo = He decided to drink it
• Decidió pedir otro = He decided to order another one

So decidió que... is about his judgment, not about a planned action.

This is a very common question.

Estaba mejor is used because the sentence is talking about the milkshake’s condition, taste, or how it seemed at that moment.

• estar mejor = to be better, to taste better, to feel better, to seem better in a particular situation

Why not era mejor?
Because ser would sound more like a general or defining statement:

• El batido era mejor sin tanto sirope could suggest a more general idea about what kind of milkshake is better.
• El batido estaba mejor sin tanto sirope focuses more on this milkshake as he experienced it.

Why not estuvo mejor?
Because estuvo would treat it as a more bounded, completed state. Here, estaba sounds more natural because it describes the state he perceived when he made his judgment.

So estaba mejor is the most natural choice for it was better / it tasted better in that moment.

Because mejor is the normal comparative form of bueno.

• bueno = good
• mejor = better

So Spanish usually says:

• está mejor
• es mejor
• sabe mejor

rather than más bueno.

You may occasionally hear más bueno in some contexts or regions, but for standard Spanish, especially in a sentence like this, mejor is the expected form.

Because tanto is used before nouns, while tan is used before adjectives or adverbs.

Here sirope is a noun, so Spanish uses:

• tanto sirope = so much syrup

Compare:

• tan dulce = so sweet
• tan rápido = so quickly
• tanto azúcar = so much sugar
• tantos vasos = so many glasses

Also, tanto agrees with the noun:

• tanto sirope because sirope is masculine singular
• tanta azúcar because azúcar is usually treated as feminine in many contexts
• tantos cubitos
• tantas fresas

Because Spanish often leaves out the article after sin when speaking about a substance or amount in a general way.

• sin azúcar
• sin sal
• sin mucho ruido
• sin tanto sirope

Adding an article would usually make it sound more specific or less natural here. In this sentence, the focus is on the quantity of syrup, not on identifying a particular syrup as a separate object.

Because el batido refers to a specific milkshake that is already known in the context.

Spanish usually needs an article in a sentence like this:

• el batido estaba mejor...

If you said un batido, that would introduce it as one milkshake among others, not the already established one. And batido with no article would not sound natural here.

So el works like the in English: it points to the specific milkshake being discussed.

Yes, sirope is a very natural word in Spain for syrup, especially in food and dessert contexts, like toppings for pancakes, ice cream, or milkshakes.

A learner might also know jarabe, but in Spain that often means syrup medicine, or sometimes a syrup in a more technical sense. For a sweet topping in a milkshake, sirope is the most natural choice.

Because Después de otro sorbo is an introductory phrase, and Spanish often sets off introductory elements with a comma.

• Después de otro sorbo, mi hermano decidió...

This helps the sentence read more clearly. In short sentences, Spanish punctuation can sometimes be flexible, so you may occasionally see this kind of comma omitted. But here the comma is perfectly natural and good style.