No quiero ese libro; prefiero el otro.

In Spanish, no normally goes directly before the conjugated verb:

• No quiero
• No tengo
• No sé

So No quiero ese libro is the standard way to say it. You do not put no after the verb in a sentence like this.

Spanish often drops subject pronouns because the verb ending already shows who the subject is.

• quiero = I want
• prefiero = I prefer

So Yo no quiero ese libro; yo prefiero el otro is possible, but it sounds more emphatic. Without yo, the sentence is more natural in most contexts.

You would add yo if you want contrast or emphasis, for example:

• Yo no quiero ese libro; yo prefiero el otro.

That can sound like I don’t want that book; I prefer the other one with extra stress on I.

Spanish demonstratives often depend on how the speaker sees the object in relation to the people involved.

In traditional Spanish:

• este = this, near the speaker
• ese = that, near the listener or not near the speaker
• aquel = that over there, farther away from both

So ese libro suggests that book, not the one closest to the speaker.

In modern usage, especially in casual speech, the distinction between ese and aquel is not always very strict, but este / ese / aquel is still the basic system learners should know.

Prefiero comes from the verb preferir. This verb is a stem-changing verb in the present tense:

• preferir
• prefiero
• prefieres
• prefiere
• preferimos
• preferís
• prefieren

The e in the stem changes to ie in stressed forms, so:

• preferir → prefiero

This is the same kind of change you see in verbs like:

• querer → quiero
• pensar → pienso

So in this sentence, both verbs are first person singular present:

• quiero
• prefiero

Because el otro means the other one — a specific alternative already understood from context.

Here, el otro really means:

• el otro libro

The noun libro is omitted because it has just been mentioned.

Compare:

• prefiero el otro = I prefer the other one
• prefiero otro = I prefer another one / a different one

So the article el makes it sound more specific: there are two identifiable options, and the speaker wants the other one.

Yes. In Spanish, determiners and adjectives must agree in gender and number with the noun.

Since libro is masculine singular, you get:

• ese libro
• el otro

If the noun were feminine singular, you would say:

• esa revista
• la otra

If it were plural:

• esos libros
• los otros

So agreement is very important here.

Yes, if the context is clear enough.

The sentence already omits libro in the second half:

• prefiero el otro = prefiero el otro libro

You could also omit it in the first half in conversation:

• No quiero ese; prefiero el otro.

That is grammatical and natural if everyone already knows you are talking about books. Still, keeping ese libro in the first clause is often clearer, especially for learners.

The semicolon links two closely related complete clauses:

• No quiero ese libro
• prefiero el otro

A semicolon works well because the second clause contrasts with and completes the idea of the first.

A full stop would also be correct:

• No quiero ese libro. Prefiero el otro.

A comma is less formal here because these are two full clauses. In careful writing, the semicolon or full stop is better.

Because el here is the article meaning the, not the pronoun él meaning he.

Compare:

• el libro = the book
• él prefiere = he prefers

So in el otro, el is simply the masculine singular definite article.

Yes, it sounds completely natural.

No quiero ese libro; prefiero el otro is a normal, clear way to contrast two options.

In everyday conversation, people might also say:

• No quiero ese libro. Prefiero el otro.
• No quiero ese; prefiero el otro.

All of these are natural. The version with prefiero sounds slightly more deliberate than simply repeating quiero, because it emphasizes preference between alternatives.