No quiero ese; prefiero el primero.

Spanish demonstratives show distance:

• este = this one, near the speaker
• ese = that one, near the listener or a bit farther away
• aquel = that one over there, farther away from both

So No quiero ese means I don’t want that one. It refers to something already understood from context.

Because ese is being used as a pronoun, meaning that one.

Compare:

• No quiero ese libro = I don’t want that book.
• No quiero ese = I don’t want that one.

The noun is omitted because it is already clear from the situation.

Here, primero is also being used like a noun phrase: the first one. In Spanish, ordinal numbers often need the definite article when they stand in for a noun.

• Prefiero el primero = I prefer the first one.

If you said only Prefiero primero, it would usually sound like I prefer first or I’d rather do something first, which is a different meaning.

They express different ideas:

• quiero = I want
• prefiero = I prefer

So:

• No quiero ese = I don’t want that one.
• Prefiero el primero = I prefer the first one.

The second part does not just reject one option; it says which option is better in the speaker’s opinion.

Because they are different verbs, and both are irregular in the present tense:

• querer → quiero
• preferir → prefiero

Both are stem-changing verbs:

• querer: e → ie
• preferir: e → ie

Examples:

• quiero, quieres, quiere...
• prefiero, prefieres, prefiere...

So the change happens in the stem, not in the ending.

Primero is the masculine singular form of first.

It changes depending on gender and number:

• el primero = the first one, masculine
• la primera = the first one, feminine
• los primeros = the first ones, masculine plural
• las primeras = the first ones, feminine plural

So if the hidden noun were feminine, you would say:

• No quiero esa; prefiero la primera.

Both come from the same word, but they are used differently.

• primer is used before a masculine singular noun
• primero is used on its own, after the noun, or in other contexts

Examples:

• el primer libro = the first book
• el libro primero = uncommon in normal speech
• el primero = the first one

In your sentence, there is no noun after it, so el primero is correct.

Not especially for grammar. It simply links two closely related ideas:

• I don’t want that one;
• I prefer the first one.

In everyday writing, many people would also use:

• No quiero ese. Prefiero el primero.
• No quiero ese, prefiero el primero.

The semicolon gives a slightly more careful or formal written style.

In modern Spanish, normally no.

Current standard spelling writes:

• este, ese, aquel
• este, ese, aquel as pronouns too

So you write:

• No quiero ese.

Older texts or older teaching materials may show ése, but that is generally no longer recommended except in very rare cases of ambiguity.

You would change both the demonstrative and the ordinal to feminine:

• No quiero esa; prefiero la primera.

Compare:

• masculine: ese / el primero
• feminine: esa / la primera

This is because Spanish words agree in gender with the noun they refer to, even when the noun is omitted.

Not here. Although querer can sometimes relate to affection, in sentences like this it means to want.

So:

• No quiero ese = I don’t want that one.

A meaning like love would usually be clear from a very different context, especially when talking about people.

A simple guide for English speakers:

• prefiero ≈ preh-FYE-roh
• quiero ≈ KYEH-roh

A few helpful points:

• ie in these words sounds like a glide, roughly ye
• the r in prefiero is a Spanish single r, a quick tap, not a strong English r
• the qu in quiero sounds like k

So:

• quiero = KYE-roh / KYEH-roh
• prefiero = preh-FYE-roh