Si este libro es complicado, buscaré otro.

Because after si meaning if, Spanish normally uses the indicative, not the subjunctive, when talking about a real or possible condition.

So:

• Si este libro es complicado, buscaré otro. = If this book is complicated, I’ll look for another one.

Using sea here would sound wrong in standard Spanish for this kind of sentence.

A very common pattern is:

• Si + present indicative, future
• Si tengo tiempo, iré.
• Si hace frío, me quedaré en casa.

Because it expresses what the speaker will do if the condition is true.

• buscaré = I will look for / I will find / I will search for

The structure is:

• If X happens/is true, I will do Y

So:

• Si este libro es complicado, buscaré otro.

You could also hear other tenses depending on meaning:

• Si este libro es complicado, busco otro. = If this book is complicated, I look for another one / I’ll get another one.
• Si este libro es complicado, voy a buscar otro. = I’m going to look for another one.

But buscaré is a very natural choice for a future result.

Because Spanish usually leaves out subject pronouns when they are already clear from the verb ending.

• buscaré already means I will look for
• so yo buscaré is usually unnecessary

Spanish is a pro-drop language, meaning pronouns are often omitted:

• Busco = I look for
• Buscaré = I will look for

You can add yo for emphasis or contrast:

• Si este libro es complicado, yo buscaré otro. This might suggest contrast, like I’ll look for another one (even if someone else doesn’t).

Otro means another one here.

The noun libro is understood, so Spanish leaves it out:

• buscaré otro = I’ll look for another one
• full version: buscaré otro libro

This is very common in Spanish when the noun is obvious:

• Quiero este. = I want this one.
• Prefiero otro. = I prefer another one.

Because otro normally does not take un before it in standard Spanish when it means another.

So you say:

• otro libro = another book
• buscaré otro = I’ll look for another one

Not:

• un otro libro ❌

This is one of those patterns English speakers need to get used to, because English uses another, while Spanish simply uses otro.

These words show distance or perspective:

• este libro = this book (near the speaker, or the one currently being discussed)
• ese libro = that book (near the listener, or less close)
• aquel libro = that book over there / that one farther away

In your sentence, este libro most naturally means this book.

A learner should know that Spanish makes a three-way distinction more clearly than English often does:

• este
• ese
• aquel

Because it has to agree with libro, which is a masculine singular noun.

Agreement in Spanish:

• libro → masculine singular
• adjective must match → complicado

Compare:

• este libro es complicado
• esta novela es complicada

So the ending changes depending on the noun.

They are similar, but not always exactly the same.

• difícil = difficult, hard
• complicado = complicated, complex, tricky

In many situations, both work:

• Este libro es difícil.
• Este libro es complicado.

But complicado often suggests that something is complex or involved, not just hard.

So in this sentence:

• complicado suggests the book may be too complex
• difícil would emphasize difficulty more generally

The comma separates the if-clause from the main clause:

• Si este libro es complicado, buscaré otro.

In Spanish, when the conditional clause comes first, a comma is commonly used. It helps readability and reflects the pause.

If the order is reversed, the comma is usually not needed:

• Buscaré otro si este libro es complicado.

So the comma is normal and helpful here.

Usually, es complicado is the better choice here.

Why?

• ser describes an inherent quality or characteristic
• estar describes a state or condition

A book being complicated is normally treated as a characteristic of the book’s content, so:

• este libro es complicado

Está complicado can exist in Spanish, but it often means something more like it’s in a difficult situation, it’s tricky right now, or it can sound more colloquial depending on context and region.

For a book, es complicado is the standard choice.

In Spain Spanish, buscaré is pronounced approximately:

• boos-kah-REH

The stress is on the last syllable because of the accent mark:

• bus-ca-ré

A few pronunciation notes:

• b in Spanish is softer than English b
• c before a sounds like k
• final é is a clear vowel sound, not like English silent e

So:

• buscaré → boos-kah-REH

Not exactly. Buscar means to look for / to search for, not to find.

So:

• buscaré otro = I’ll look for another one

If you want I’ll find another one, the verb would usually be encontrar:

• Si este libro es complicado, encontraré otro.

That said, in natural English, I’ll get another one might sometimes be used as a smoother translation, depending on context. But the Spanish verb itself is look for/search for.