En la contraportada pone que el libro ganó un premio, pero a mí me convenció más el primer capítulo.

Contraportada means back cover of a book.

A few related words:

• portada = front cover
• contraportada = back cover

So En la contraportada... means On the back cover...

In Spanish, especially in everyday use in Spain, poner is very common for written text.

So:

• Aquí pone... = It says here...
• En la contraportada pone que... = On the back cover it says that...

This is very natural when talking about:

• signs
• labels
• instructions
• packaging
• book covers

You could sometimes hear dice, but pone sounds especially natural when referring to something physically written somewhere.

There is no clearly stated subject, and that is normal.

Spanish often uses pone in an impersonal way when talking about written text:

• Aquí pone que abre a las ocho.
• En la etiqueta pone sin azúcar.

It works a bit like English it says..., where it does not really refer to a specific thing in a very concrete way.

So in En la contraportada pone que..., the idea is simply:

• it says on the back cover that...

Ganó is the preterite, used for a completed action in the past: the book won an award at some point.

So:

• ganó = won

This is a straightforward past fact.

Could ha ganado also appear? Sometimes yes, depending on context and how connected the event feels to the present, especially in Spain. But here ganó is very natural because it presents the award as a completed past event.

Because primero becomes primer before a singular masculine noun.

This is called apocopation.

So:

• el primero = the first one
• el primer capítulo = the first chapter

Other common examples:

• el primer día
• el tercer libro from tercero

This is very common in Spanish. The phrase a mí is used for emphasis, contrast, or clarity, while me is the object pronoun that the verb normally needs.

So:

• me convenció = it convinced me
• a mí me convenció = as for me, it convinced me / it convinced me, personally

In this sentence, a mí adds contrast:

• the back cover says the book won a prize,
• but I was more convinced by the first chapter.

So the full structure gives extra emphasis to me.

Here, me is a direct object.

The verb is convencer a alguien = to convince someone.

Examples:

• El libro convenció a Marta.
• El libro la convenció.
• El libro me convenció.

So in your sentence:

• el primer capítulo = the subject
• me = the direct object
• a mí = an emphasized version of that same object

This matters because learners sometimes assume me is indirect whenever it means to me, but with convencer, it is direct.

Literally, it means convinced me more, but in natural English it often means something like:

• won me over more
• persuaded me more
• made a stronger impression on me

Here the idea is not necessarily logical argument alone. It means the first chapter was more effective at making the speaker think well of the book.

So me convenció más el primer capítulo suggests:

• the prize mentioned on the back cover was less persuasive,
• the actual first chapter was what really made the speaker believe in the book's quality.

Más means more, so it implies a comparison.

The comparison is not stated with a full second part, but it is understood from the context. The meaning is roughly:

• the fact that the book won a prize was somewhat convincing,
• but the first chapter convinced me more

So más is comparing the first chapter with the information on the back cover, or more generally with other reasons to think the book is good.

Spanish often leaves this kind of comparison implicit when it is clear from context.

Yes, El primer capítulo me convenció más is also correct.

The version in your sentence places a mí first to highlight the speaker's personal reaction:

• pero a mí... = but as for me...

Then el primer capítulo comes later, which gives a natural rhythm and keeps the contrast strong.

So the chosen word order helps emphasize:

1. contrast with the previous idea
2. the speaker's own opinion

Spanish word order is often more flexible than English, especially when emphasis is involved.

Because pero introduces a contrast, not a correction of a negative statement.

Here the meaning is:

• the back cover says one thing,
• but the speaker was persuaded more by something else

So pero is right.

You use sino after a negative statement when you replace one idea with another:

• No ganó un premio, sino dos.
• No me convenció la portada, sino el primer capítulo.

Since your sentence is not structured as not X, but Y, pero is the correct conjunction.

Not naturally in this case.

After pone, Spanish normally uses a full clause with que when reporting what the text says:

• pone que el libro ganó un premio
• pone que abre a las nueve
• pone que no se admiten devoluciones

So que introduces the content of what is written.

Using an infinitive here would not sound natural for this meaning.

Yes, it sounds very natural in Spain.

Using poner to mean to say/write is very common in Spain in contexts involving written text:

• En el cartel pone...
• En la web pone...
• En la caja pone...

Learners who know mostly Latin American Spanish may notice that decir can feel more expected to them, but poner is absolutely standard and very common in Spain for written information.