En la pantalla aparecían el origen, el destino y la puerta de embarque.

Because the sentence is describing what was visible on the screen as part of the background of a scene. In this context, aparecían means something like were shown, were appearing, or could be seen.

The imperfect is common when Spanish describes:

• an ongoing situation in the past
• what things were like
• what was visible, happening, or present at that time

If you used aparecieron instead, it would sound more like the information appeared at a specific moment.

Because together they form a compound subject: three separate things are doing the action of appearing on the screen.

So:

• el origen = singular
• el destino = singular
• la puerta de embarque = singular

But:

• el origen, el destino y la puerta de embarque = plural idea overall

That is why the verb is aparecían, not aparecía.

Spanish often places the verb before the subject, especially when introducing or listing information. So aparecían el origen, el destino y la puerta de embarque is very natural.

You could also say:

El origen, el destino y la puerta de embarque aparecían en la pantalla.

That is also correct, but it sounds a bit more marked or less neutral in this kind of description. The original sentence flows naturally as a description of what was on the screen.

Because Spanish usually uses en where English often uses on with screens, pages, signs, and similar surfaces.

So:

• en la pantalla = on the screen
• en la página = on the page
• en el cartel = on the sign/poster

This is a very common difference between English and Spanish. It is best learned as a normal Spanish pattern rather than translated word for word.

Yes, but in Spanish it is often used in a broader way than English appear. In this sentence, aparecer can mean:

• to appear
• to show up
• to be displayed
• to be visible

So En la pantalla aparecían... is a natural way to say that those pieces of information were displayed on the screen.

Spanish often uses definite articles more than English does, especially when referring to specific, identifiable items.

Here, the sentence is talking about specific pieces of information shown on the screen:

• el origen
• el destino
• la puerta de embarque

Repeating the articles makes the list clear and natural. Spanish does this very often in lists. English is more likely to drop repeated articles in similar cases, but Spanish usually keeps them.

It means boarding gate.

Literally:

• puerta = door / gate
• embarque = boarding

So puerta de embarque is the gate where passengers go to board the plane.

In airport Spanish, this is the standard term in Spain. Even though English uses gate, Spanish commonly uses puerta.

Because standard Spanish normally does not use the Oxford comma.

So the natural punctuation is:

el origen, el destino y la puerta de embarque

not usually:

el origen, el destino, y la puerta de embarque

Spanish generally only adds a comma before y if there is a special reason, such as avoiding confusion or marking an interruption.

Yes. That is also correct, but the nuance is slightly different.

• En la pantalla aparecían... focuses on what could be seen or what appeared on the screen.
• La pantalla mostraba... focuses more directly on what the screen displayed.

Both are natural. The original version is a little more descriptive and scene-setting, while mostraba can sound a bit more direct.

Not necessarily wrong, but it would change the meaning.

• aparecían = the information was there / was visible / was being shown
• aparecieron = the information appeared at a particular moment

So if you are describing the screen as part of a scene, aparecían is the better choice. If you want to emphasize that the information suddenly came up or appeared, aparecieron would make sense.