Al principio del viaje, mi hermana estaba nerviosa, pero ahora está más tranquila.

In Spanish, al principio is the normal idiomatic way to say at the beginning.

• a + el = al
• So al principio literally comes from a el principio

Native speakers usually say:

• Al principio del viaje... = At the beginning of the trip...

En el principio is much less common and often sounds unnatural here. It can appear in special contexts, but for everyday Spanish, al principio is the phrase you should learn.

Del is the contraction of de + el.

• de el viaje → del viaje

You use de here because Spanish expresses the beginning of the trip as the beginning of the trip, literally:

• el principio del viaje = the beginning of the trip

So:

• al principio del viaje = at the beginning of the trip

Because the sentence contrasts a past state with a present state.

• estaba = imperfect past of estar = was
• está = present of estar = is

So:

• mi hermana estaba nerviosa = my sister was nervous
• ahora está más tranquila = now she is calmer

The speaker is describing how she felt earlier in the trip and how she feels now.

Estaba is the imperfect, which is often used for background descriptions, ongoing states, and how someone was feeling.

Here, nerviosa is a state or feeling, so estaba fits well:

• Al principio del viaje, mi hermana estaba nerviosa
  = At the beginning of the trip, my sister was nervous

If you used estuvo, it would sound more like you are treating the nervousness as a completed event or bounded period. That is possible in some contexts, but estaba is more natural for simply describing her emotional state at that time.

Because nerviosa and tranquila describe temporary emotional states or conditions here.

In Spanish, estar is commonly used for:

• feelings
• moods
• temporary conditions
• states that can change

So:

• estar nerviosa = to be nervous
• estar tranquila = to be calm

Using ser here would usually sound wrong, because ser is more for identity, essential characteristics, or more permanent traits.

Because they agree with mi hermana, which is feminine singular.

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

• mi hermano estaba nervioso = my brother was nervous
• mi hermana estaba nerviosa = my sister was nervous

And similarly:

• tranquilo for a masculine singular noun
• tranquila for a feminine singular noun

Más tranquila means calmer or more calm, so it shows a comparison.

The sentence implies:

• she was nervous before
• now she is calmer than before

So:

• ahora está más tranquila = now she is calmer / more relaxed

If you said only ahora está tranquila, it would mean now she is calm, which is also possible, but más tranquila highlights the change from her earlier state.

Because the comparison is understood from the context.

In Spanish, you use más ... que ... when you say both sides of the comparison explicitly:

• Está más tranquila que antes = She is calmer than before

But if the second part is obvious, Spanish can leave it out:

• Ahora está más tranquila = Now she is calmer

The unstated idea is than before.

Pero means but, and ahora means now.

Together they create a contrast:

• before: estaba nerviosa
• now: está más tranquila

So pero ahora marks the change very clearly:

• ..., pero ahora... = ..., but now...

It helps organize the sentence into two time periods: earlier and now.

Yes, but it changes the meaning slightly.

• estaba nerviosa = she was nervous
  
  • describes her state
• se puso nerviosa = she became nervous / got nervous
  
  • focuses on the change into that state

So:

• Al principio del viaje, mi hermana estaba nerviosa
  describes how she was at the beginning

• Al principio del viaje, mi hermana se puso nerviosa
  suggests that at the beginning of the trip, something caused her to become nervous

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

Yes. Depending on context, tranquila can mean:

• calm
• relaxed
• less anxious
• at ease

In this sentence, más tranquila could be understood as:

• calmer
• more relaxed
• less nervous

That is why it works naturally as the opposite of nerviosa.

Because Al principio del viaje is an introductory time phrase, and the comma helps separate it from the main clause.

So the structure is:

• Al principio del viaje, = introductory time expression
• mi hermana estaba nerviosa = main clause

Then there is another comma before pero, which is also normal when joining two clauses with pero:

• ..., pero ahora está más tranquila.

This punctuation makes the sentence easier to read and is standard Spanish writing.