Mi hermana se pone nerviosa al despegar, pero sonríe en cuanto el avión empieza a aterrizar.

Ponerse often means to become or to get. So se pone nerviosa means she gets nervous or she becomes nervous.

That is slightly different from está nerviosa, which would simply describe her state: she is nervous.

So:

• Mi hermana se pone nerviosa al despegar = my sister gets nervous at takeoff
• Mi hermana está nerviosa = my sister is nervous

Here, ponerse highlights the change into that emotional state.

In ponerse, the se is part of the pronominal verb ponerse. You learn it as a whole verb.

• poner = to put
• ponerse = to become / to get / to put on, depending on context

In this sentence, ponerse nerviosa is a very common structure meaning to get nervous.

It is not reflexive in the literal English sense of put herself here. It is simply how Spanish expresses this change of state.

Because it describes mi hermana, and hermana is feminine singular.

Adjectives in Spanish usually agree with the noun they describe:

• mi hermano está nervioso
• mi hermana está nerviosa

So here:

• Mi hermana se pone nerviosa

If the subject were masculine, it would be nervioso.

Al + infinitive is a very common Spanish structure meaning when, upon, or sometimes while doing something.

So:

• al despegar = when taking off / at takeoff
• literally: on taking off

In this sentence, it means that her nervousness happens at the moment of takeoff.

Other examples:

• Al entrar, saludó a todos. = When he came in, he greeted everyone.
• Al verlo, sonrió. = Upon seeing him, she smiled.

Yes, despegar can mean to unstick or to detach, but with planes it also means to take off.

So in an aviation context:

• El avión despega = the plane takes off
• El avión aterriza = the plane lands

Spanish often uses the same verb in different contexts, and the meaning is clear from the situation.

Both can work, but they are not exactly the same.

• al despegar = when taking off / at the moment of takeoff
• durante el despegue = during takeoff

Al despegar focuses more on the action as it begins or happens. Durante el despegue focuses more on the whole period of takeoff.

In this sentence, al despegar sounds very natural and concise.

The accent mark in sonríe shows the stress and helps keep the vowels pronounced separately.

The verb is sonreír = to smile.

Its forms include:

• sonrío
• sonríes
• sonríe

The accent helps indicate that í is stressed, so it is pronounced roughly as separate vowel sounds rather than blending into one syllable.

Spanish often leaves out subject pronouns because the verb ending usually makes the subject clear.

So instead of saying:

• Ella sonríe

Spanish very often just says:

• Sonríe

In your sentence, the subject is already clear from Mi hermana, so repeating ella would usually be unnecessary.

En cuanto means as soon as.

So:

• sonríe en cuanto el avión empieza a aterrizar = she smiles as soon as the plane starts to land

It introduces the moment something happens.

Other examples:

• Te llamo en cuanto llegue. = I’ll call you as soon as I arrive.
• En cuanto lo vio, lo entendió. = As soon as she saw it, she understood it.

Empezar a + infinitive means to start doing something.

So:

• empieza a aterrizar = starts to land
• aterriza = lands / is landing

The sentence specifically focuses on the beginning of the landing process, not the whole landing.

So there is a small difference:

• en cuanto el avión aterriza = as soon as the plane lands
• en cuanto el avión empieza a aterrizar = as soon as the plane starts to land

The second one happens earlier.

Because empezar normally uses a before another verb in the infinitive:

• empezar a hablar = to start speaking
• empezar a correr = to start running
• empezar a aterrizar = to start landing

This is just the standard pattern:

• empezar a + infinitive

Yes, en cuanto can be followed by either indicative or subjunctive depending on the meaning.

Here it uses the indicative, empieza, because the sentence describes a real, habitual situation: this is what happens when she flies.

• Mi hermana se pone nerviosa..., pero sonríe en cuanto el avión empieza a aterrizar.

This sounds like a repeated fact.

If you were talking about a future event, Spanish would usually use the subjunctive:

• Mi hermana sonreirá en cuanto el avión empiece a aterrizar.

So:

• indicative = real, habitual, known situation
• subjunctive = future, uncertain, not yet realized event

The basic verb for to smile is sonreír, without se.

• sonríe = she smiles

There is also sonreírse in some contexts, but it is not the basic neutral form and often adds a nuance depending on region or usage. For a standard sentence like this, sonríe is the normal choice.

So:

• Mi hermana sonríe = my sister smiles

Not necessarily. In Spanish, the present tense can describe:

• what is happening now
• habitual actions
• general patterns

Here it most naturally describes a habitual pattern:

• whenever the plane takes off, she gets nervous
• whenever it starts landing, she smiles

So the sentence means something like this in natural English:

• My sister gets nervous at takeoff, but smiles as soon as the plane starts to land.

It is not limited to one specific flight happening right now.

Yes, often the subject can be omitted if it is obvious from context.

For example:

• Nos ponemos nerviosos al despegar. = We get nervous at takeoff.
• En cuanto empieza a aterrizar, se tranquiliza. = As soon as it starts landing, she calms down.

But in your sentence, el avión is included to make the second part very clear:

• en cuanto el avión empieza a aterrizar

That explicitly tells us what is starting to land.