Resto in biblioteca fino alle otto.

Because restare means to stay / to remain, while essere means to be.

So Resto in biblioteca means I stay in the library, not just I am in the library.
Using resto emphasizes remaining there for a period of time.

• Resto in biblioteca = I stay in the library
• Sono in biblioteca = I am in the library

Both can be correct in different situations, but they do not mean exactly the same thing.

Resto is the first person singular present tense of restare.

The present tense forms are:

• io resto = I stay
• tu resti = you stay
• lui/lei resta = he/she stays
• noi restiamo = we stay
• voi restate = you all stay
• loro restano = they stay

In this sentence, resto means I stay or I’m staying.

Italian often leaves out subject pronouns when they are already clear from the verb form.

Since resto clearly means I stay, you do not need io.

• Resto in biblioteca = I stay in the library
• Io resto in biblioteca = I stay in the library

Both are correct, but adding io usually gives extra emphasis, contrast, or clarity.

For example:

• Io resto, tu vai = I’m staying, you’re leaving

With many places, Italian uses in to mean in/at a place, especially when talking about being inside or spending time there.

So:

• in biblioteca = in / at the library
• in ufficio = in / at the office
• in casa = at home / in the house

Alla biblioteca can sometimes appear, but it usually sounds more like to the library or refers to a more specific directional/contextual use. In this sentence, in biblioteca is the natural choice for staying in/at the library.

Not in this sentence.

Italian often does not use an article after certain prepositions when talking about common places or institutions in a general way:

• in biblioteca = in/at the library
• a scuola = at school
• in ufficio = at the office

So Resto in biblioteca is perfectly natural.

In other contexts, you can find an article, especially when the noun is more specific:

• nella biblioteca comunale = in the municipal library
• nella biblioteca dell’università = in the university library

Fino a means until or up to.

When a comes before the plural feminine article le, they combine:

• a + le = alle

Since otto here refers to the eight o’clock hour, Italian uses le otto.
So:

• fino a + le otto = fino alle otto

This means until eight o’clock.

Because when telling time in Italian, you usually use the article:

• l’una = one o’clock
• le due = two o’clock
• le otto = eight o’clock

So at eight is alle otto, not a otto.

Examples:

• Arrivo alle otto = I arrive at eight
• Lavoro fino alle otto = I work until eight

In Italian, clock times from 2 onward are treated with the plural article le:

• le due
• le tre
• le otto

Only one o’clock is singular:

• l’una

So alle otto is standard because otto is part of the time expression le otto.

Yes. The Italian present tense often covers both the simple present and the present continuous, depending on context.

So Resto in biblioteca fino alle otto can mean:

• I stay in the library until eight
• I’m staying in the library until eight

English chooses between those forms more strictly than Italian does.

The normal word order here is very natural:

• Resto in biblioteca fino alle otto.

But Italian word order can be flexible for emphasis.

For example:

• Fino alle otto resto in biblioteca. = Until eight, I’m staying in the library.
• In biblioteca resto fino alle otto. = In the library, I stay until eight.

These alternatives are possible, but the original sentence is the most neutral and standard.

Yes. Rimango from rimanere can also mean I stay / I remain.

So both are possible:

• Resto in biblioteca fino alle otto
• Rimango in biblioteca fino alle otto

In many everyday situations, restare is slightly more common and straightforward for learners, but rimanere is also very natural Italian.

A simple pronunciation guide is:

REH-stoh een bee-blee-oh-TEH-kah FEE-noh AHL-leh OT-toh

A few useful notes:

• resto: the r is rolled or tapped
• biblioteca: stress is on te
• fino: stress is on fi
• otto: the double tt is pronounced more strongly than a single t
• alle: stress is on the first syllable

If you want to sound natural, keep the rhythm smooth and link the words together: Resto in biblioteca fino alle otto.