A capa do livro é bonita, mas o índice me ajudou mais do que eu esperava.

Because Portuguese nouns have grammatical gender.

• capa is a feminine noun, so it takes a
• índice is a masculine noun, so it takes o

This grammatical gender does not always match any logical idea of male/female. It is just something you learn with the noun.

That is also why the adjective changes in é bonita:

• capa → feminine
• so the adjective becomes bonita, not bonito

do is a contraction of de + o.

• de = of / from
• o = the
• de + o → do

So a capa do livro literally means the cover of the book.

Portuguese uses these contractions very often:

• de + a → da
• de + os → dos
• de + as → das

Because adjectives usually agree with the noun they describe in gender and number.

Here, bonita describes capa, and capa is feminine singular. So the adjective must also be feminine singular:

• a capa bonita
• o livro bonito
• as capas bonitas
• os livros bonitos

So A capa do livro é bonita uses bonita because it refers to capa, not livro.

Both are possible in Portuguese, but in Brazilian Portuguese, me ajudou is much more natural and common in everyday speech.

• me ajudou = normal in Brazil
• ajudou-me = correct, but sounds more formal, literary, or European Portuguese-like

Here me means me as the object of the verb:

• ajudar alguém = to help someone
• me ajudou = helped me

So o índice me ajudou literally means the index helped me.

mais do que means more than.

So:

• mais = more
• do que = than

Together:

• mais do que eu esperava = more than I expected

This is a very common comparison pattern in Portuguese:

• mais do que = more than
• menos do que = less than
• tão... quanto / como = as... as

Examples:

• Foi mais fácil do que eu pensei. = It was easier than I thought.
• Ela trabalhou mais do que o irmão. = She worked more than her brother.

This is a very important tense contrast.

• ajudou is the preterite: a completed action in the past
• esperava is the imperfect: something ongoing, backgrounded, or describing a mental state in the past

So the sentence presents:

• the index helped me as a completed event
• I expected as an ongoing expectation or state of mind at that time

This is very natural in Portuguese. Compare:

• Ele chegou quando eu dormia.
• She/He arrived when I was sleeping.

In your sentence:

• o índice me ajudou = the helping happened as a finished fact
• eu esperava = what I had been expecting

Yes, Portuguese often drops subject pronouns, and this sentence could also be:

• ...mais do que esperava.

That would still be correct.

But eu can be included for reasons like:

• slight emphasis
• clarity
• rhythm of the sentence
• personal contrast, even if subtle

So both are possible:

• mais do que eu esperava
• mais do que esperava

In Brazilian Portuguese, people do use subject pronouns quite a lot, even when they are not strictly necessary.

In context, índice often refers to the table of contents, though depending on the book it can also mean an index.

This can be a little tricky because Portuguese usage is not always identical to English publishing terminology.

In Brazilian Portuguese:

• índice can refer to a list of contents or references
• sumário is also very common for table of contents

So if you are thinking in natural book terms, many Brazilians would probably understand this sentence as something like:

• The table of contents helped me more than I expected or
• The index helped me more than I expected

The exact translation depends on the kind of book.

mas means but.

It connects two contrasting ideas:

• A capa do livro é bonita = The book’s cover is beautiful
• mas o índice me ajudou mais do que eu esperava = but the index helped me more than I expected

So the contrast is:

• the cover is attractive
• but the really useful thing was the index

It is a very common conjunction in Portuguese.

The accents are part of the correct spelling and pronunciation.

• é is the verb ser in the present: is
• The accent marks it as a stressed vowel and also distinguishes it from e, which means and

So:

• é = is
• e = and

In índice, the accent shows the stress falls early in the word:

• ÍN-di-ce

Without getting too technical, the accent helps show how the word is pronounced and written correctly.

Yes, but it changes the nuance.

• é bonita suggests the cover is beautiful as a quality or characteristic
• está bonita suggests it looks beautiful now, in the current state or appearance

In many situations, both can work, but é bonita is the more natural choice when describing the cover in a general way.

So:

• A capa do livro é bonita. = The cover is beautiful.
• A capa do livro está bonita. = The cover looks beautiful / is looking nice.

Mostly yes. Ajudar usually works like to help and can take a direct object:

• ajudar alguém = to help someone
• O índice me ajudou. = The index helped me.

You may also see:

• ajudar a fazer algo = help to do something
• me ajudou a entender o livro = helped me understand the book

So in your sentence, me is the person receiving the help, just as in English helped me.