Pienso pasar por la biblioteca antes de volver a casa.

Because pensar + infinitive often means to intend to or to be planning to do something.

So:

• Pienso pasar por la biblioteca = I’m planning to stop by the library
• not necessarily I think about stopping by the library

Compare:

• Pienso que es buena idea = I think it’s a good idea
• Pienso ir mañana = I plan to go tomorrow

This is a very common structure in Spanish.

Pienso is the present indicative of pensar.

Spanish often uses the present tense to talk about plans, intentions, or near-future actions, especially when the context makes the future meaning clear.

So Pienso pasar... is similar to:

• I plan to stop by...
• I’m thinking of stopping by...

It does not mean the action is happening right now. It means the speaker currently has that intention.

Because Spanish usually leaves out subject pronouns when they are obvious from the verb form.

• Pienso already tells you it means I think / I plan
• so yo is optional

You could say Yo pienso pasar por la biblioteca..., but that would usually add emphasis, contrast, or clarity.

For example:

• Yo pienso pasar por la biblioteca, pero ella no.
  = I’m planning to stop by the library, but she isn’t.

Without emphasis, Pienso... is the most natural.

Here pasar por means something like:

• to stop by
• to drop by
• to go via
• to pass by

In this sentence, the most natural sense is to stop by the library, probably briefly and on the way somewhere else.

That is different from simply going to the library as the main destination. Pasar por often suggests a quick visit or a stop along the route.

Yes, it can. Pasar por can sometimes mean literally to pass by a place.

But in a sentence like this, many learners will understand it as to stop by the library, especially because it sounds like part of a plan before going home.

So the exact nuance depends on context:

• Pasé por la biblioteca could mean I stopped by the library
• or I passed by the library

If the speaker wants to make it crystal clear that they are going inside, they might say something more explicit, such as ir a la biblioteca or mention what they are doing there.

They are similar, but not identical.

• Ir a la biblioteca = to go to the library
• Pasar por la biblioteca = to stop by / go by the library

Pasar por often suggests:

• the visit may be brief
• it may be one stop among others
• it may happen on the way somewhere else

Since the sentence continues with antes de volver a casa, pasar por fits very naturally: the speaker plans to stop by the library before going home.

Because in Spanish, when antes is followed by an infinitive, you normally use de:

• antes de + infinitive

So:

• antes de volver = before returning
• antes de comer = before eating
• antes de salir = before leaving

This is a fixed and very common pattern.

In Spanish, casa often appears without an article when it means home.

So:

• volver a casa = to return home
• llegar a casa = to arrive home
• estar en casa = to be at home

If you say la casa, it usually sounds more like the house/building, not home in the general sense.

So:

• Volver a casa = go back home
• Volver a la casa = go back to the house

The second one is possible, but it has a more literal, physical sense.

Because the verb volver normally uses a when it means to return to a place.

So:

• volver a casa = return home
• volver al trabajo = return to work
• volver a Madrid = return to Madrid

That a is the normal preposition used with destinations.

Yes, because learners sometimes mix up biblioteca and librería.

• biblioteca = library
• librería = bookshop / bookstore

So in this sentence, la biblioteca definitely means the library, not a place where books are sold.

This is a very common vocabulary trap.

It can mean either, depending on context.

Volver basically means to return. English often chooses between:

• come back
• go back
• return

In antes de volver a casa, the most neutral translation is before going back home or before returning home.

Spanish does not always separate come back and go back as strictly as English does.

Yes. Regresar a casa is also correct and means the same thing: to return home.

However, volver is extremely common and very natural in everyday Spanish.

So both work:

• antes de volver a casa
• antes de regresar a casa

In many contexts, volver sounds a bit more everyday and conversational.

A few natural alternatives are:

• Voy a pasar por la biblioteca antes de volver a casa.
  = I’m going to stop by the library before going home.

• Quiero pasar por la biblioteca antes de volver a casa.
  = I want to stop by the library before going home.

• Tengo pensado pasar por la biblioteca antes de volver a casa.
  = I’m planning to stop by the library before going home.

The original sentence is already natural. It simply uses pensar + infinitive to express intention.

It breaks down like this:

• Pienso = I plan / I intend
• pasar por la biblioteca = to stop by the library
• antes de = before
• volver a casa = returning home / going back home

So the structure is:

pensar + infinitive
plus
antes de + infinitive

That makes it a very useful model sentence for building similar ones, such as:

• Pienso estudiar antes de salir.
• Pienso llamar a mi madre antes de acostarme.
• Pienso comprar pan antes de volver a casa.