La chica silbó una canción mientras esperaba el autobús.

This is a very common Spanish tense contrast:

• silbó = preterite: a completed action, seen as a whole
• esperaba = imperfect: an ongoing background action

So the sentence presents the situation like this:

• She whistled a song = the main event
• while she was waiting for the bus = the ongoing background activity

Spanish often uses this pattern:

• preterite for the main event
• imperfect for what was happening around it

Mientras means while.

It does not always require the imperfect, but it very often introduces an action that was in progress when something else happened. That is why esperaba sounds natural here.

Examples:

• Mientras estudiaba, sonó el teléfono.
  While I was studying, the phone rang.
• Mientras comíamos, empezó a llover.
  While we were eating, it started to rain.

If both actions are viewed as complete events, other tenses can also appear.

In Spanish, the definite article is used more often than in English.

• la chica = the girl

If you just said chica by itself in this sentence, it would sound incomplete. Spanish normally needs an article or another determiner before a singular countable noun:

• la chica = the girl
• una chica = a girl
• esa chica = that girl

Una canción means a song.

The sentence is talking about one song, but not a specific one already known to the listener, so Spanish uses the indefinite article:

• una canción = a song
• la canción = the song

So:

• silbó una canción = she whistled a song / she whistled a tune

For the same reason as la chica: Spanish usually needs an article before a singular count noun.

• el autobús = the bus

Also, in Spanish (including Spain), transport nouns often appear with the article:

• tomé el tren = I took the train
• esperaba el autobús = I was waiting for the bus

Yes. Silbar means to whistle, and it can be used with a direct object:

• silbar una canción = to whistle a song
• silbar una melodía = to whistle a tune

That structure is perfectly normal in Spanish.

Not exactly.

• niña usually means girl in the sense of a child
• chica can mean girl, young woman, or just female person depending on context

So la chica often sounds more like:

• the girl
• the young woman
• the girl/young woman in question

It is a very common everyday word in Spain.

Because Spanish verb endings usually show who the subject is.

• esperaba can mean I was waiting, he/she was waiting, or you (formal) were waiting, depending on context
• Here, the subject is clearly la chica, so ella is unnecessary

Spanish often omits subject pronouns when the subject is already clear:

• La chica esperaba el autobús.
• Ella esperaba el autobús.

Both are possible, but the first is more natural unless you want emphasis or contrast.

The accent marks help show stress and, in some cases, distinguish forms.

• silbó: the accent shows stress on the last syllable, and it also identifies this as the third-person singular preterite form

  • silbo = I whistle
  • silbó = he/she whistled

• canción: stress falls on the final syllable -ción

• autobús: stress falls on the final syllable -bús

Accent marks are important in Spanish because they can change pronunciation and sometimes meaning or grammar.

The basic structure is:

• La chica = subject
• silbó = main verb
• una canción = direct object
• mientras esperaba el autobús = time clause

So:

La chica | silbó | una canción | mientras esperaba el autobús

This is a very standard and natural word order in Spanish.

Yes, but it changes the structure a bit.

• La chica silbó una canción mientras esperaba el autobús.
• La chica silbó una canción mientras estaba esperando el autobús.

Both are understandable. However, Spanish often prefers the simple imperfect esperaba instead of estaba esperando when giving background information in a past narrative.

So esperaba is usually the more natural choice here.

Yes, autobús is standard and very common in Spain.

You may also hear:

• bus in some contexts, especially informal or influenced by English
• guagua in places like the Canary Islands
• other regional terms in the Spanish-speaking world

But for standard Spain Spanish, autobús is an excellent choice.

Not if you want to keep the same meaning.

• silbar = to whistle
• cantar = to sing

So:

• La chica silbó una canción = The girl whistled a song
• La chica cantó una canción = The girl sang a song

They are different actions.