Mi hermana no sabía qué souvenir elegir, así que al final compró una postal y una taza.

Because sabía is the imperfect of saber, and here it describes an ongoing situation in the past: she didn’t know which souvenir to choose.

• No sabía... = she didn’t know / she wasn’t sure
• No supo... would sound more like she failed to find out or she couldn’t figure out at a specific moment

In this sentence, the speaker is describing her state of uncertainty, so imperfect is the natural choice.

It has an accent because it is an interrogative word in an embedded question.

Here, qué souvenir elegir means something like what/which souvenir to choose. Even though it is not a direct question with question marks, it still contains a question idea internally.

Compare:

• No sabía qué elegir. = She didn’t know what to choose.
• El souvenir que eligió... = The souvenir that she chose...

In the second example, que has no accent because it is a relative word, not a question word.

In Spanish, qué is normally used before a noun:

• qué souvenir
• qué libro
• qué color

Cuál is usually used on its own, not directly before a noun:

• ¿Cuál eliges? = Which one are you choosing?
• No sabía cuál elegir. = She didn’t know which one to choose.

So in this sentence:

• qué souvenir elegir = which souvenir to choose

That is the normal pattern.

Because after sabía qué..., Spanish uses the infinitive to express what to do or which one to choose.

So:

• No sabía qué souvenir elegir = She didn’t know which souvenir to choose

This is very common in Spanish:

• No sé qué hacer = I don’t know what to do
• No sabía dónde ir = She didn’t know where to go
• No sabemos cuándo salir = We don’t know when to leave

Because Spanish often drops subject pronouns when the verb ending already shows who the subject is.

• sabía
• compró

Both forms clearly refer to she here, especially since Mi hermana has already been mentioned.

So Spanish prefers:

• Mi hermana no sabía... así que al final compró...

Instead of:

• Mi hermana no sabía... así que ella al final compró...

Using ella is possible, but it would usually add emphasis or contrast.

Así que means so, so then, or therefore.

It connects the two ideas:

• she didn’t know which souvenir to choose
• so in the end she bought a postcard and a mug

It is very common in everyday Spanish.

Examples:

• Tenía hambre, así que comí algo. = I was hungry, so I ate something.
• Era tarde, así que nos fuimos. = It was late, so we left.

Here al final means in the end or eventually.

• Al final compró una postal y una taza. = In the end, she bought a postcard and a mug.

It is similar to finalmente, but not always identical in tone.

• al final often sounds more conversational and can suggest after some hesitation or after considering options
• finalmente can sound a little more formal or narrative

In this sentence, al final fits very naturally because she was unsure and then eventually made a decision.

Yes, souvenir is understood and used in Spain, especially in tourist contexts. However, a more traditionally Spanish word is recuerdo.

So both are possible:

• un souvenir
• un recuerdo

In Spain, you will definitely hear and see souvenir, especially in shops. A learner should recognize it easily, but it is also useful to know recuerdo.

Yes. Elegir and escoger both mean to choose.

So you could also say:

• Mi hermana no sabía qué souvenir escoger...

In most contexts, they are interchangeable. Elegir is extremely common and sounds completely natural here.

Spanish normally repeats the article with each singular noun when listing them:

• una postal y una taza

That is the most natural structure.

English often allows:

• a postcard and mug

But Spanish strongly prefers:

• una postal y una taza

Repeating the article makes the sentence sound complete and natural.

Because in Spanish, basic negation is formed by putting no directly before the verb.

• no sabía = did not know
• no compró = did not buy

So:

• Mi hermana no sabía...

is the standard way to say My sister didn’t know...

Yes, grammatically that is correct:

• Mi hermana no sabía qué recuerdo elegir...

However, there is a small nuance:

• souvenir strongly suggests a tourist item
• recuerdo can also mean memory, although in this context it would still be understood as souvenir

So souvenir sounds especially natural if the context is travel or gift shops.