Una vez que ella llegó, comimos.

Here una vez que means once / after / once that in the sense of after something happened.

So Una vez que ella llegó, comimos means:

• Once she arrived, we ate
• After she arrived, we ate

Even though una vez by itself can mean one time or once, in una vez que it works as a fixed time expression introducing another clause.

Because both actions are presented as completed events in the past.

• llegó = she arrived
• comimos = we ate

The sentence describes a sequence:

1. she arrived
2. then we ate

Spanish commonly uses the preterite for this kind of finished, time-bounded event.

If the sentence were describing background or an ongoing situation, Spanish might use the imperfect instead, but that is not what is happening here.

Yes, it could be omitted.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is:

• llegó already tells us it is he/she/it arrived
• so Una vez que llegó, comimos is perfectly natural

Including ella can add:

• emphasis
• contrast
• clarity if there are several possible people being discussed

So:

• Una vez que llegó, comimos = neutral
• Una vez que ella llegó, comimos = a bit more explicit, possibly emphasizing she

Yes. That is very natural too.

• Cuando ella llegó, comimos = When she arrived, we ate
• Una vez que ella llegó, comimos = Once/after she arrived, we ate

The difference is mainly nuance:

• cuando simply places the two events in time
• una vez que emphasizes that the second action happened after the first was completed

So una vez que can sound slightly more like once she had arrived, then we ate.

Both can be used in similar contexts, but the nuance is a little different:

• Una vez que ella llegó, comimos = Once/after she arrived, we ate
• En cuanto llegó, comimos = As soon as she arrived, we ate

So:

• una vez que focuses on completion of the first action
• en cuanto emphasizes immediacy

If you want the idea of immediately after, en cuanto is often stronger.

Because the sentence begins with a subordinate time clause:

• Una vez que ella llegó = introductory clause
• comimos = main clause

In Spanish, it is normal to put a comma after an introductory subordinate clause like this.

If you reverse the order, you would usually not use a comma:

• Comimos una vez que ella llegó.

Both are grammatically fine, but the original order is often more natural when setting the time first.

It literally means we ate, but depending on context it can also mean we had lunch or we had a meal.

In Spain, comer often refers specifically to the main midday meal, so in the right context comimos may be understood as we had lunch.

Without more context, the safest basic meaning is simply we ate.

The accent mark shows both pronunciation and verb form.

• llegó = he/she arrived (preterite, 3rd person singular)
• llego = I arrive (present, 1st person singular)

So the accent is important because it helps distinguish different forms.

In llegó, the stress falls on the last syllable: lle-GÓ.

Sometimes yes, but not in this sentence.

In Una vez que ella llegó, comimos, the arrival is a completed past fact, so Spanish uses the indicative:

• llegó

But if the action is in the future or not yet completed, Spanish often uses the subjunctive:

• Una vez que ella llegue, comeremos. = Once she arrives, we’ll eat.

So a useful rule is:

• completed/real event → usually indicative
• future/pending event → often subjunctive

Yes. You can also say:

• Comimos una vez que ella llegó.

This is grammatically correct, but the focus changes slightly.

• Una vez que ella llegó, comimos puts the time/background first
• Comimos una vez que ella llegó puts the main action first

Both mean essentially the same thing, but the original version is often a more natural way to frame the sequence of events.