Il tempo passa in fretta quando leggo un libro interessante.

In Italian, nouns often take an article where English might not use one. Il tempo means time here, and Italian normally says the time in this kind of general statement.

So:

• Il tempo passa = Time passes
• not usually just Tempo passa

This is very common in Italian with general ideas and abstract nouns.

Passa is the third-person singular form of the verb passare in the present tense.

Here, the subject is il tempo, which is singular, so the verb must also be singular:

• il tempo passa = time passes

Present tense endings for passare:

• io passo
• tu passi
• lui/lei passa
• noi passiamo
• voi passate
• loro passano

Both can work, but they are not used in exactly the same way.

• in fretta is a very common expression meaning quickly / fast / in a hurry
• velocemente also means quickly, but it can sound a bit more literal or formal depending on context

In a sentence like this, Italians very naturally say:

• Il tempo passa in fretta

This is one of the most common ways to express time goes by quickly.

Quando means when and introduces a subordinate clause.

The sentence has two parts:

• Il tempo passa in fretta = main clause
• quando leggo un libro interessante = subordinate clause

So quando links the two ideas:

• time passes quickly
• when I read an interesting book

Leggo is the first-person singular present tense of leggere (to read).

Because the subject is I, Italian uses:

• (io) leggo = I read / I am reading

Italian often uses the simple present where English might use either:

• I read
• I am reading

So in this sentence, leggo is perfectly natural.

Sto leggendo would mean I am reading and is more specifically progressive:

• Il tempo passa in fretta quando sto leggendo un libro interessante

That is possible, but the original sentence is more general and more idiomatic.

Italian usually drops subject pronouns when they are clear from the verb form.

Since leggo already tells you the subject is I, io is not necessary.

So:

• quando leggo = when I read

You could say quando io leggo, but that would usually add emphasis or contrast, for example:

• quando io leggo, non mi disturbare = when I’m reading, don’t disturb me

In neutral sentences, Italian normally omits io.

The masculine singular indefinite article is usually:

• un

So:

• un libro
• un film
• un ragazzo

Uno is used before certain sounds, such as:

• s + consonant: uno studente
• z: uno zaino
• gn: uno gnomo
• ps: uno psicologo

Since libro begins with a normal l sound, it takes un:

• un libro interessante

In Italian, adjectives often come after the noun.

So:

• un libro interessante = an interesting book

That is the normal order here.

Italian sometimes puts adjectives before the noun, but this can change the tone or emphasis. For a straightforward description, libro interessante is the standard choice.

Yes, adjectives must agree with the noun in gender and number. However, interessante is one of those adjectives that has the same form for masculine and feminine singular.

Examples:

• un libro interessante = masculine singular
• una storia interessante = feminine singular
• libri interessanti = masculine plural
• storie interessanti = feminine plural

So the singular form stays interessante, but the plural becomes interessanti.

Yes, absolutely. That is also correct.

Italian word order is flexible enough that both are natural:

• Il tempo passa in fretta quando leggo un libro interessante.
• Quando leggo un libro interessante, il tempo passa in fretta.

The meaning stays the same. The difference is mostly about emphasis:

• starting with Il tempo passa in fretta emphasizes the result first
• starting with Quando leggo... emphasizes the situation first

By default, it sounds more like a general truth or a habitual situation:

• Time passes quickly when I read an interesting book

The present tense in Italian often expresses general situations like this.

If you wanted one specific past moment, you would normally use a past tense, for example:

• Il tempo è passato in fretta quando ho letto un libro interessante.

So the original sentence feels general, not tied to one single event.

Yes, tempo can mean both time and weather in Italian. Context tells you which one is meant.

Here, it clearly means time because of the phrase:

• Il tempo passa in fretta = Time passes quickly

If it meant weather, you would see different contexts, for example:

• Che tempo fa? = What’s the weather like?
• Il tempo è bello oggi. = The weather is nice today.

So in this sentence, tempo definitely means time.

You would change both the noun and the article:

• libro → libri
• usually un libro would become just libri in a general plural statement

So:

• Il tempo passa in fretta quando leggo libri interessanti.

Notice the adjective also changes:

• interessante → interessanti

because it must agree with the plural noun libri.