Tropecé con la mochila de mi hermana al entrar en la biblioteca.

Because the verb is tropezar. In Spanish, verbs ending in -zar change z to c before e in some forms to preserve the sound.

So:

• tropezar
• yo tropecé = I tripped

This is the same pattern as:

• empezar → empecé
• almorzar → almorcé

The accent on -é shows that it is 1st person singular preterite: I tripped.

Tropecé is the preterite, which is used for a completed action at a specific moment in the past.

Here, the speaker is talking about one event:

• Tropecé... = I tripped...

If you said tropezaba, that would usually suggest an ongoing, repeated, or background action, which does not fit as naturally here.

Compare:

• Tropecé con la mochila = I tripped over the backpack once.
• Tropezaba mucho de niño = I used to trip a lot as a child.

Because the expression is tropezar con when you mean to trip over / bump into something.

So:

• tropezar con algo
• tropecé con la mochila = I tripped over the backpack

This is just how the verb is commonly built in Spanish. A learner often wants to translate directly from English and expects something else, but con is normal here.

Also note that tropezar con can sometimes mean to run into / come across in a more figurative sense:

• Tropecé con un viejo amigo = I ran into an old friend

So context matters.

Often yes, but not always exactly.

In this sentence, tropezar con la mochila naturally means to trip over the backpack.

But tropezar con can also mean:

• to bump into
• to run into
• to come across unexpectedly

So it has a slightly broader range than English trip over.

Here the physical meaning is clear because of la mochila.

Because Spanish commonly expresses possession with de:

• la mochila de mi hermana = my sister’s backpack

This is often more common and more neutral than using a possessive adjective for everything.

You could compare:

• la mochila de mi hermana = my sister’s backpack
• su mochila = her backpack

But su mochila could also mean his backpack, your backpack, their backpack, etc., depending on context. So de mi hermana is clearer.

You could, if the context already makes it clear whose backpack it was.

But su in Spanish can mean:

• his
• her
• your
• its
• their

So la mochila de mi hermana is more specific and avoids ambiguity.

It can also sound more natural when you want to identify the person clearly, especially if they have not been mentioned immediately before.

Al + infinitive is a very common Spanish structure. It usually means:

• when doing
• upon doing
• as someone was doing

So:

• al entrar = when entering / on entering / as I was entering

In this sentence:

• Tropecé con la mochila... al entrar en la biblioteca
• I tripped over my sister’s backpack when I was entering the library

This structure is very natural in Spanish and often sounds smoother than a full clause like cuando entré.

Yes. That would also be correct:

• Tropecé con la mochila de mi hermana cuando entré en la biblioteca.

That means roughly the same thing.

The difference is mostly style:

• al entrar is more compact and elegant
• cuando entré is more explicit and finite

Both are common. Spanish often prefers al + infinitive for actions happening right at the moment another action occurs.

It is understood to be the same subject as the main verb unless context strongly suggests otherwise.

So here:

• Tropecé... al entrar...
• The person who tripped is also the person entering the library.

That is the normal interpretation.

In other words, I tripped ... as I was entering the library.

Good question. With entrar, both en and a can appear, but they are not always used in exactly the same way.

Very generally:

• entrar en often emphasizes being inside the place
• entrar a can also be used, especially in some varieties, with a sense of entering a place or going in to do something

In Spain, entrar en la biblioteca is very normal and standard.

So this sentence is completely natural:

• al entrar en la biblioteca

You may also hear:

• entrar a la biblioteca

But entrar en is especially common and safe to use.

Because after entrar, Spanish often uses en to mark the place entered:

• entrar en la casa
• entrar en la habitación
• entrar en la biblioteca

By contrast, ir a la biblioteca means to go to the library, focusing on the destination.

So:

• ir a la biblioteca = go to the library
• entrar en la biblioteca = enter the library / go into the library

They describe different stages of movement.

Because in Spanish, specific places usually take the definite article:

• la biblioteca
• el colegio
• la casa
• el banco

So en la biblioteca means in the library / into the library.

Spanish uses articles more often than English does. English might sometimes say at school or in prison without an article, but Spanish often keeps the article where English may drop it.

Yes, Spanish word order is fairly flexible, though some orders sound more natural than others.

The original sentence is very natural:

• Tropecé con la mochila de mi hermana al entrar en la biblioteca.

You could also say:

• Al entrar en la biblioteca, tropecé con la mochila de mi hermana.

This version puts the time/action setting first: As I was entering the library, I tripped over my sister’s backpack.

Both are correct. The second one slightly foregrounds the moment of entering.

Yes, mochila is the normal and very common word for backpack in Spain.

Depending on context, you might also hear other words in the Spanish-speaking world, but mochila is standard and widely understood everywhere.

In Spain, this sentence sounds perfectly natural with mochila.

The stress falls on the last syllable: tro-pe-CÉ.

The written accent is important because it shows where the stress goes and helps identify the tense/form:

• tropecé = I tripped

Without the accent, it would not be spelled correctly.

So the accent is not optional here; it is part of the correct written form.

Not usually. Al + infinitive normally means something like:

• when
• as
• upon

So al entrar en la biblioteca suggests the action happened at the moment of entering or while entering, not well after.

If you wanted after entering, you would more naturally say something like:

• después de entrar en la biblioteca

So in this sentence, the idea is that the tripping happened right around the moment of going in.

Because tropezar is not reflexive in this meaning.

You say:

• tropezar con algo = to trip over something

So:

• Tropecé con la mochila = I tripped over the backpack

English often uses I tripped, and Spanish does the same without a reflexive pronoun here.

A learner may expect me tropecé, but that is not correct standard usage for this meaning.