Ya tengo escrita la reseña de la novela, pero quiero hacerla un poco más corta.

Ya often means already, but its exact feel depends on context.

In Ya tengo escrita la reseña..., it tells you that the review is done at this point or I’ve got it written already. It often adds the idea that this is a completed step before the next one:

• Ya tengo escrita la reseña = I already have the review written
• then: pero quiero hacerla... = but I want to make it...

So ya is marking progress: one thing is finished, but another change is still wanted.

These two forms are related, but they do not mean exactly the same thing.

• He escrito la reseña = I have written the review
  • This focuses on the action of writing.
• Tengo escrita la reseña = I have the review written / I’ve got the review written
  • This focuses on the resulting state: the review exists in a written form now.

So tener + past participle often emphasizes that something is done and available as a result.

Here, Ya tengo escrita la reseña sounds natural because the speaker is talking about the review as something already completed, and now they want to edit it.

Because escrita agrees with la reseña, which is a feminine singular noun.

The participle escrito/escrita/escritos/escritas behaves like an adjective here, so it must agree in gender and number with the thing described:

• el informe escrito
• la reseña escrita
• los artículos escritos
• las cartas escritas

In this sentence:

• reseña = feminine singular
• so: escrita

Both can be understood, but de is the more natural choice here.

• la reseña de la novela = the review of the novel
• la reseña sobre la novela = the review about the novel

In Spanish, reseña de... is the usual pattern for reviews:

• una reseña de una película
• una reseña de un libro
• una reseña de una novela

So de la novela is the most idiomatic phrasing.

The -la is a direct object pronoun meaning it, and it refers to la reseña.

So:

• quiero hacerla = I want to make it
• la = the review

Spanish often uses object pronouns attached to an infinitive:

• quiero leerlo = I want to read it
• voy a comprarla = I’m going to buy it
• necesito corregirla = I need to correct it

Because reseña is feminine singular, the pronoun is la.

In Spanish, when you have a conjugated verb plus an infinitive, the object pronoun can often go:

1. attached to the infinitive, or
2. before the conjugated verb

So both are correct:

• Quiero hacerla un poco más corta
• La quiero hacer un poco más corta

The version with the pronoun attached to the infinitive is very common and often sounds smoother.

Yes. In Spanish, hacer + object + adjective is a very common way to say make something + adjective.

So:

• hacerla más corta = make it shorter
• hacerlo más claro = make it clearer
• hacerla más interesante = make it more interesting

Even though the review is already written, the speaker wants to edit it so that it becomes shorter.

Because corta agrees with la reseña, which is feminine singular.

Here, corta describes the review:

• la reseña es corta
• quiero hacerla más corta

Since reseña is feminine, the adjective must also be feminine:

• corto for masculine singular
• corta for feminine singular

Compare:

• Quiero hacer el resumen más corto
• Quiero hacer la reseña más corta

Un poco means a little or a bit.

So:

• más corta = shorter
• un poco más corta = a little shorter / a bit shorter

It softens the statement and makes it sound less extreme. The speaker does not want to reduce it drastically, just a small amount.

Yes, that is grammatically correct, and it sounds natural. But the nuance changes slightly.

• Ya he escrito la reseña... focuses on the fact that the writing action has been completed.
• Ya tengo escrita la reseña... focuses more on the finished review as something now available and ready to revise.

In this context, tengo escrita works especially well because the speaker immediately talks about editing it.

The original order is natural, but there is some flexibility.

Standard version:

• Ya tengo escrita la reseña de la novela, pero quiero hacerla un poco más corta.

Possible variation:

• La reseña de la novela ya la tengo escrita, pero quiero hacerla un poco más corta.

That version adds emphasis to la reseña de la novela.

However, for a learner, the original sentence is the best model because it is neutral and very idiomatic.

Because once la reseña has been mentioned, Spanish usually replaces it with a pronoun if it remains the object.

So instead of saying:

• ...pero quiero hacer la reseña un poco más corta

Spanish naturally says:

• ...pero quiero hacerla un poco más corta

This avoids repetition, just like English often says make it shorter instead of make the review shorter.

It is a structure where tener is followed by a past participle that agrees with the object. It often expresses a resulting state, something like to have something done/written/prepared.

Examples:

• Tengo preparada la cena = I have dinner ready
• Tengo hechas las tareas = I have the homework done
• Tiene leídos muchos libros de Cervantes = He/She has many Cervantes books read

In your sentence:

• tengo escrita la reseña
• literally: I have the review written
• natural English: I already have the review written

This is a very useful pattern in Spanish.