Quiero pasar por la biblioteca antes de volver a casa.

Because after querer (to want), Spanish normally uses an infinitive for the next verb when the subject stays the same.

• Quiero pasar = I want to stop by / go by
• Quiero volver = I want to return

This is the same basic pattern as English want to + verb.

If the subject changes, Spanish often uses que + a conjugated verb instead:

• Quiero pasar = I want to stop by
• Quiero que pases = I want you to stop by

Spanish often leaves out subject pronouns because the verb ending already shows who is doing the action.

• quiero already means I want
• so yo quiero is usually unnecessary

You can add yo for emphasis, contrast, or clarity:

• Yo quiero pasar por la biblioteca, pero él no.

In a neutral sentence, Quiero pasar... sounds completely natural.

In this sentence, pasar por means something like:

• to stop by
• to drop by
• to go by

It does not necessarily mean entering for a long time. It often suggests a brief visit or going via that place.

Depending on context, pasar por can also mean literally to pass by a place, so context matters. Here, with la biblioteca, learners will often understand it as stop by the library or go by the library.

Because pasar por is a fixed expression.

• ir a la biblioteca = to go to the library
• pasar por la biblioteca = to stop by / go by the library

So the preposition comes from the verb phrase itself:

• pasar por + place

You usually cannot swap por for a here without changing the meaning.

In Spanish, places often appear with a definite article when talking about a specific, understood location.

• la biblioteca
• el banco
• la farmacia

So pasar por la biblioteca sounds natural as stop by the library.

English sometimes drops or keeps the differently from Spanish, so article use does not always match word-for-word.

Because in Spanish, antes de is the normal structure before an infinitive.

• antes de volver
• antes de salir
• antes de comer

So the pattern is:

• antes de + infinitive

If a full clause follows, Spanish usually uses antes de que + subjunctive:

• antes de que vuelva
• antes de que salgamos

Because a casa is a very common fixed expression meaning home.

• volver a casa = to go back home / return home
• llegar a casa = to get home
• estar en casa = to be at home

Spanish usually does not say a la casa when it simply means home in a general sense.

But a la casa can appear when you mean a specific house as a building:

• Volví a la casa de Ana. = I returned to Ana’s house.

Because volver means to return / go back, which fits the idea of going back home after doing something else.

• ir a casa = to go home
• volver a casa = to go back home / return home

So volver tells you the speaker is already out somewhere and plans to return home afterward.

Yes. Regresar a casa is grammatically correct and means basically the same thing.

• volver a casa
• regresar a casa

However, volver is extremely common in everyday Spanish and often sounds a bit more natural in casual speech. In Spain, volver is especially frequent.

Yes. Spanish allows some flexibility in word order.

For example:

• Antes de volver a casa, quiero pasar por la biblioteca.
• Quiero pasar por la biblioteca antes de volver a casa.

Both are natural. The second version is very neutral. The first puts more focus on the time sequence.

Not exactly.

• ir a la biblioteca = to go to the library
• pasar por la biblioteca = to stop by / go by the library

Ir a simply says the library is the destination.
Pasar por often suggests a quick stop or that it is one point along the way.

So the choice depends on the nuance the speaker wants.

Yes: the stress falls on -te-.

It is pronounced roughly like:

• bi-blio-TE-ca

That is because words ending in a vowel are normally stressed on the second-to-last syllable in Spanish unless there is a written accent mark. Since biblioteca has no accent mark, the normal stress rule applies.

No, not with the same meaning.

• antes de volver a casa = before returning home
• para volver a casa = in order to return home / to return home

So antes de expresses time, while para usually expresses purpose.

That is why antes de volver a casa is the correct choice here.