Appena apro il libro, mi metto a sfogliare le pagine che ho già evidenziato.

Here appena means as soon as.

So Appena apro il libro means As soon as I open the book.

This is a very common use of appena before a verb. Be careful, because appena can also mean just / only just in other contexts, for example:

• Ho appena mangiato = I’ve just eaten

But in your sentence, it clearly means as soon as.

Apro is the present tense of aprire.

Italian often uses the present tense to talk about:

• habitual actions
• repeated actions
• vivid narration

So Appena apro il libro, mi metto... sounds like:

• Whenever I open the book, I start... or
• As soon as I open the book, I start...

It does not have to refer only to the exact present moment.

If you said ho aperto, that would mean I opened / I have opened, which would change the structure completely.

Because the verb here is mettersi a + infinitive, which is a very common Italian structure meaning:

• to begin to
• to start to

So:

• mi metto a sfogliare = I start leafing through / I begin to leaf through

The mi is the reflexive pronoun that goes with mettersi.

Full pattern:

• mi metto
• ti metti
• si mette
• ci mettiamo
• vi mettete
• si mettono

Even though English does not say I put myself to..., Italian does.

It means to start doing something.

In this sentence:

• mi metto a sfogliare = I start leafing through

It often gives a sense of suddenly or actively beginning an action.

Examples:

• Si è messo a ridere = He started laughing
• Mi metto a studiare = I start studying
• Ci mettiamo a lavorare = We start working

A close equivalent is iniziare a, but mettersi a can sound a little more vivid or action-oriented.

Sfogliare usually means to leaf through, to flip through, or to thumb through pages.

So:

• sfogliare il libro = to leaf through the book
• sfogliare le pagine = to flip through the pages

It often suggests looking through pages quickly, not necessarily reading every word carefully.

That makes it a very natural verb in this sentence.

Both are possible.

• sfogliare il libro = leaf through the book
• sfogliare le pagine = leaf through the pages

In your sentence, le pagine is useful because the relative clause che ho già evidenziato describes the pages specifically. The speaker is not just flipping through the whole book at random, but through the pages that they have already highlighted.

So le pagine is more precise here.

Che is a relative pronoun meaning that / which.

It refers back to le pagine:

• le pagine che ho già evidenziato = the pages that I have already highlighted

So the structure is:

• le pagine = the pages
• che = that / which
• ho già evidenziato = I have already highlighted

This is very similar to English relative clauses.

Because the highlighting happened before the action of flipping through the pages.

The logic is:

1. I previously highlighted some pages.
2. As soon as I open the book, I start leafing through those already-highlighted pages.

So Italian uses the passato prossimo:

• ho evidenziato = I highlighted / I have highlighted

The word già strengthens this idea of prior completion:

• already

So:

• che ho già evidenziato = that I have already highlighted

Because evidenziare takes avere in compound tenses.

So:

• ho evidenziato
• hai evidenziato
• ha evidenziato

This is normal because evidenziare is a transitive verb: you highlight something.

For example:

• Ho evidenziato il titolo = I highlighted the title

Già means already.

It tells you that the pages were highlighted before the current action started.

Compare:

• le pagine che ho evidenziato = the pages that I highlighted
• le pagine che ho già evidenziato = the pages that I have already highlighted

So già emphasizes that this is something done earlier, not right now.

Because the two actions have different time relationships.

• Appena apro il libro → present tense, describing what happens whenever this situation occurs
• mi metto a sfogliare → present tense, the next action in that same habitual sequence
• che ho già evidenziato → past action completed before the page-flipping

So the sentence mixes:

• a habitual present sequence with
• a previous completed action

This is very natural in Italian, just as it is in English:

• As soon as I open the book, I start leafing through the pages that I’ve already highlighted.

Here appena is followed by the indicative, which is normal in this kind of time clause:

• Appena apro il libro...

Italian commonly uses the indicative after appena when it means as soon as.

So this sentence is completely standard.

Yes, absolutely.

• mi metto a sfogliare = I start leafing through
• inizio a sfogliare = I begin to leaf through

Both are correct.

The difference is mostly stylistic:

• iniziare a is more neutral
• mettersi a can feel a bit more dynamic, as if the person gets into the action right away

So the original sentence sounds lively and natural.

Not exactly, though they are related.

• evidenziare usually means to highlight, often with a highlighter
• sottolineare means to underline

So:

• ho evidenziato le pagine suggests the pages contain highlighted text
• ho sottolineato le frasi suggests the speaker underlined sentences

In everyday speech, learners should keep this distinction in mind.

The comma separates the introductory time clause from the main clause:

• Appena apro il libro, = introductory clause
• mi metto a sfogliare... = main clause

This is also common in English:

• As soon as I open the book, I start...

In short, the comma helps readability and is perfectly normal here.