Aunque estaba nerviosa, mi hermana dejó de tartamudear en cuanto vio a la profesora sonreír.

Because estaba is the imperfect, which often gives background information or describes an ongoing state.

Here, aunque estaba nerviosa means she was feeling nervous at that moment. It sets the scene for the main action.

Using estuvo would sound more like a completed, bounded state, which is less natural in this context.

So the contrast is:

• estaba nerviosa = she was nervous / she felt nervous
• dejó de tartamudear = the main event that happened

This is a very common pattern in Spanish: imperfect for background, preterite for the main event.

Because nerviosa agrees with mi hermana, which is feminine singular.

In Spanish, adjectives usually agree in gender and number with the noun they describe:

• mi hermano estaba nervioso
• mi hermana estaba nerviosa

So the ending changes because the person being described is female.

This is the structure:

• dejó = she stopped / she quit
• de + infinitive = stopped doing something

So:

• dejar de + infinitive = to stop doing something

Examples:

• dejó de fumar = she stopped smoking
• dejé de pensar en eso = I stopped thinking about that
• mi hermana dejó de tartamudear = my sister stopped stammering/stuttering

This is a very common and useful structure in Spanish.

Yes. Tartamudear is the standard verb meaning to stutter or to stammer.

You may also see the noun:

• la tartamudez = stutter/stammering

In everyday use, tartamudear is perfectly normal and natural.

En cuanto means as soon as.

After en cuanto, Spanish often uses a normal past tense when talking about something that actually happened in the past:

• en cuanto vio... = as soon as she saw...

So the sequence is:

1. she was nervous
2. she saw the teacher smile
3. she stopped stammering

Because this is a completed past event, vio is in the preterite.

Yes, but the meaning changes slightly.

• cuando vio = when she saw
• en cuanto vio = as soon as she saw

En cuanto is more immediate. It emphasizes that one thing happened right after the other.

So in this sentence, en cuanto helps show that the teacher’s smile immediately triggered the change.

Because vio presents the seeing as a single completed event.

• vio = she saw
• veía = she was seeing / used to see / would see, depending on context

Here, the sentence is about a specific moment: she saw the teacher smile, and then she stopped stammering. That calls for the preterite:

• en cuanto vio a la profesora sonreír

If you used veía, it would suggest a more ongoing or repeated action, which does not fit as well here.

That is the personal a.

In Spanish, when the direct object is a specific person, you usually put a before it.

So:

• vio a la profesora = she saw the teacher

Compare:

• vio la película = she saw the film
• vio a la profesora = she saw the teacher

Even though la profesora is the direct object, Spanish still adds a because it refers to a person.

Because Spanish often uses this pattern:

• ver + a + person + infinitive

It means to see someone do something.

So:

• vio a la profesora sonreír = she saw the teacher smile

This is very common:

• vi a Marta entrar = I saw Marta come in
• oí al niño llorar = I heard the child cry
• miré a la gente correr = I watched people run

You could sometimes use a different structure, but ver + a + person + infinitive is the most straightforward and natural here.

Because sonreír here is linked directly to vio in the pattern see someone do something.

• vio a la profesora sonreír = she saw the teacher smile

Spanish generally prefers the infinitive after perception verbs like:

• ver = to see
• oír = to hear
• sentir = to feel/hear

So while sonriendo means smiling, it is not the usual choice in this structure.

It can mean either, depending on context.

In Spanish, profesor/profesora can refer to a teacher, and in some contexts also to a professor. In a school-type sentence like this, English will often translate it simply as the teacher.

Since the sentence uses la profesora, we know the teacher is female.

Aunque means although / even though.

It introduces a contrast:

• she was nervous
• nevertheless, she stopped stammering

So the sentence structure is:

• Aunque estaba nerviosa = although she was nervous
• mi hermana dejó de tartamudear... = my sister stopped stammering...

This is a very common way to express contrast in Spanish.

Yes. Spanish word order is flexible.

For example, these are also possible:

• Mi hermana, aunque estaba nerviosa, dejó de tartamudear en cuanto vio a la profesora sonreír.
• En cuanto vio a la profesora sonreír, mi hermana dejó de tartamudear, aunque estaba nerviosa.

The original version sounds very natural because it first sets up the contrast with aunque estaba nerviosa, then gives the main action.

Spanish often drops subject pronouns like yo, ella, nosotros, because the verb ending already shows who is doing the action.

But mi hermana is not just a pronoun — it is a full noun phrase. It is included here to identify exactly who the subject is.

Without it, dejó could mean he/she/you formal stopped, so mi hermana makes the sentence clear.

Yes, that is strongly implied.

If someone stopped doing something, the natural assumption is that they were doing it before.

So dejó de tartamudear suggests that she had been stammering and then stopped.

That implication comes naturally from the structure dejar de + infinitive.

Yes. Everything in the sentence is fully natural in Spain Spanish.

Nothing here is unusual or strongly regional:

• aunque
• estaba nerviosa
• dejó de tartamudear
• en cuanto
• vio a la profesora sonreír

All of these are standard Spanish and would sound normal in Spain.