Mi hermana y yo nos pasamos la tarde en la biblioteca.

Both can be understood, but Mi hermana y yo is the more natural and polite order in Spanish, just like my sister and I is usually preferred over I and my sister in English.

Spanish often puts yo later in a list, especially out of politeness.

Nos means ourselves/us, but in this sentence it is part of the expression pasarse + time expression.

So:

• pasar la tarde = to spend the afternoon
• pasarse la tarde = to spend the afternoon, often with a slightly stronger sense of ending up spending it or spending a long time doing that

Here, nos agrees with Mi hermana y yo, because the subject is we.

Both are possible, but they are not exactly the same in tone.

• Pasamos la tarde en la biblioteca = We spent the afternoon in the library.
• Nos pasamos la tarde en la biblioteca = We spent the afternoon in the library, with a slightly more emphatic or idiomatic feel.

Using pasarse with time expressions is very common in Spanish. It can suggest that the whole period was taken up by that activity or place.

So the reflexive form here is not really translated word-for-word into English, but it is perfectly natural in Spanish.

Literally, pasarse comes from pasar, meaning to pass. So nos pasamos la tarde is roughly we passed the afternoon.

In natural English, though, you would normally say we spent the afternoon.

This is a good example of a phrase that should be learned as a chunk:

• pasarse la mañana
• pasarse la tarde
• pasarse el día
• pasarse la noche

In Spanish, when talking about parts of the day in expressions like spend the afternoon, the definite article is normally used:

• la mañana = the morning
• la tarde = the afternoon/evening
• la noche = the night
• el día = the day

So Spanish says:

• pasar la tarde
• pasar la mañana
• pasar el día

where English often has no article in equivalent expressions like all afternoon or this morning.

Pasamos is the preterite form of pasar for nosotros.

The preterite is used because the sentence refers to a completed event in the past: the afternoon was spent, and that action is viewed as finished.

So:

• nos pasamos = we spent
• specifically, we spent it and that period is complete

If the sentence were describing a habitual action in the past, Spanish might use the imperfect instead:

• Mi hermana y yo nos pasábamos la tarde en la biblioteca. = My sister and I used to spend the afternoon in the library.

Yes, very possibly.

The phrase nos pasamos la tarde en la biblioteca often suggests that the afternoon was largely or entirely spent there. Depending on context, a natural English translation could be:

• My sister and I spent the afternoon in the library.
• My sister and I spent all afternoon in the library.

The exact nuance depends on context, but the Spanish often gives a sense of a substantial stretch of time.

Because nos is an unstressed object/reflexive pronoun, and with a conjugated verb it normally goes before the verb:

• nos pasamos
• me quedo
• se fueron

Pronouns are attached to the end only in certain cases, such as:

• infinitives: pasarnos
• gerunds: pasándonos
• affirmative commands: pasaos

So in this sentence, nos pasamos is the normal placement.

Yes. That sentence is also correct and natural.

The difference is mainly one of style and nuance:

• pasamos la tarde = neutral, straightforward
• nos pasamos la tarde = more idiomatic/emphatic, often suggesting we ended up spending the whole afternoon there

In many contexts, the two are very close in meaning.

Because en indicates location: where the afternoon was spent.

• en la biblioteca = in the library

If you used a la biblioteca, that would normally suggest movement toward the library:

• Fuimos a la biblioteca = We went to the library.

So:

• nos pasamos la tarde en la biblioteca = we spent the afternoon in the library
• fuimos a la biblioteca = we went to the library

In Spain, la tarde usually covers the period after lunch and before night, so it can overlap with what English sometimes calls afternoon and sometimes early evening.

That means the exact English translation may depend on context. Often afternoon is the best choice, but in some situations the time frame may feel a bit later than in English.

Yes, grammatically you could say:

• Nosotros nos pasamos la tarde en la biblioteca.

But Spanish often omits subject pronouns unless they are needed for emphasis or clarity. Also, if you want to specify who we are, then Mi hermana y yo is more informative.

So the original sentence is more natural if the speaker wants to make the subject explicit.

Yes, it uses the pronominal verb pasarse.

However, this is not a typical reflexive idea like I wash myself. The pronoun does not really mean that the subject is doing the action to itself in a literal way.

Instead, pasarse here is part of an idiomatic construction used with time expressions. So it is better to think of pasarse la tarde as a set phrase rather than trying to translate nos literally every time.

You could say something like:

• Mi hermana y yo estuvimos toda la tarde en la biblioteca.

That means My sister and I were in the library all afternoon.

But that is slightly different from:

• Mi hermana y yo nos pasamos la tarde en la biblioteca.

The version with pasarse la tarde focuses on how the afternoon was spent, while estuvimos focuses more simply on being there.

Both are possible, but the original is more idiomatic for expressing how time was spent.