Como empezó a llover al llegar a la estación, hubo que esperar bajo la marquesina hasta que abrieron el torniquete de al lado.

Because como here means since / because, not how.

• como = since / because when it introduces a cause, especially at the start of a sentence
• cómo = how in questions or exclamations

So Como empezó a llover... means Since it started raining...

A very common pattern in Spanish is:

• Como + cause, result

For example:

• Como era tarde, nos fuimos. = Since it was late, we left.

This is the verb pattern empezar a + infinitive, which means to start/begin to do something.

So:

• empezó a llover = it started to rain

The a is required by the verb pattern. Other examples:

• empezó a hablar = he/she started speaking
• empezaron a correr = they started running

You cannot normally drop the a here.

Because llover is an impersonal weather verb in Spanish. Weather verbs usually appear in the 3rd person singular without a real subject.

So:

• llueve = it rains / it is raining
• llovió = it rained
• empezó a llover = it started to rain

Spanish does not need a dummy subject like English it.

al + infinitive is a very common structure meaning:

• when ...
• upon ...
• on ...-ing

So:

• al llegar a la estación = when arriving at the station / upon arriving at the station

It is shorter and more compact than saying:

• cuando llegamos a la estación

Both are possible, but al + infinitive is very natural in Spanish.

Because llegar normally takes the preposition a before a destination.

So:

• llegar a casa
• llegar al hotel
• llegar a la estación

This is just how the verb works in Spanish. English learners often want to leave out the a, but it is normally needed.

They are in the preterite because the sentence describes completed events in a sequence:

• it started raining
• there was a need to wait
• they opened the nearby turnstile

The preterite is very common for events that happened and moved the story forward.

Compare:

• empezó = it started
• hubo que esperar = we/people had to wait
• abrieron = they opened

If the sentence were describing background or an ongoing situation, the imperfect might appear instead, but here the actions are presented as specific events.

haber que + infinitive is an impersonal structure meaning:

• it was necessary to ...
• one had to ...
• people had to ...

So:

• hubo que esperar = it was necessary to wait / we had to wait / people had to wait

It does not directly name who had to wait. The context tells you.

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

• hubo que esperar = impersonal; it focuses on the situation and the necessity
• tuvimos que esperar = personal; it clearly means we had to wait

So hubo que esperar sounds a bit more general and less personal. It is often used in narration when the speaker does not want to emphasize who had to do it.

bajo means under / beneath.

So:

• bajo la marquesina = under the canopy/shelter

Here bajo is very natural because we are talking about standing under a structure that gives cover from rain.

You could also hear debajo de, but bajo is shorter and very idiomatic in this kind of sentence.

In Spain, marquesina usually means a covered structure such as a:

• canopy
• shelter
• awning

In transport contexts, it can be the roofed area that protects people from rain, for example at a bus stop, station entrance, or platform area.

So here it means they waited under some kind of station shelter.

Because hasta que is used when a full clause with a conjugated verb follows.

So:

• hasta que abrieron el torniquete = until they opened the turnstile

If you use an infinitive, the structure is different:

• hasta abrir... would not work here with this meaning

Also, the verb after hasta que is in the preterite because the waiting ended when that completed action happened: they opened it.

The subject is omitted because Spanish often leaves out subject pronouns and even noun subjects when they are understood from context.

Here abrieron means:

• they opened

But they does not have to be stated. It probably refers to:

• station staff
• transport employees
• the people in charge

Spanish often does this when the exact identity is not important.

torniquete means turnstile.

de al lado means:

• next to it
• beside it
• the one nearby

So el torniquete de al lado is:

• the turnstile next to the other one
• the nearby turnstile

This expression is very common:

• la puerta de al lado = the next door / the door next to it
• el bar de al lado = the bar next door

Because Spanish is a pro-drop language: subject pronouns are often omitted when the verb form or the context already makes the meaning clear.

For example:

• hubo que esperar does not need nosotros
• abrieron does not need ellos

In fact, adding pronouns everywhere often sounds unnatural in Spanish unless you want emphasis or contrast.

So leaving them out is normal, especially in narration.